Replace :! binding syntax with phase keywords and contextual defaults (#7479)

Implement the toolchain side of proposal #7254, removing the `:!`
binding
syntax for generic and template parameters in favor of the keywords
`generic`,
`template`, and `runtime` plus contextual defaults for phase.

For valid programs this is semantics-preserving: each binding resolves
to the
same phase, and produces the same SemIR, as it did under `:!`/`:`. The
parser
derives a binding's phase from its syntactic context plus any explicit
phase
keyword; new diagnostics and error recovery for misused keywords are
described
below.

Implementation details for each component:

- Lexer: remove the `:!` (`ColonExclaim`) token, move its virtual
parse-node
  budget onto `:`, and add the `generic` and `runtime` keywords.
- Parser: thread a `BindingContext` (`ExplicitParam`, `DeducedParam`, or
`CompileTimeEntityParam`) from declaration introducers down through
parameter
lists to each binding pattern, using a one-token lookahead to
distinguish a
name-qualifier parameter list from a declaration's own final list.
Parameters
of a compile-time entity (`class`, `interface`, `constraint`, `choice`,
`alias`, `export`, `namespace`) and deduced `[]` parameters default to
checked
generic; explicit function parameters and local bindings default to
runtime.
`HandleBindingPattern` resolves the phase from that context plus the
keyword: a
`generic` keyword needs no node of its own (the phase is carried by the
  binding's node kind), while a `runtime` keyword is preserved as a
`RuntimeBindingName` node so `check` can name it in a diagnostic. A
phase
keyword that is merely redundant with the contextual default is
diagnosed
  here, without invalidating the parse tree.
- Check: a phase keyword that is invalid for its context (for example
`runtime`
on a checked-generic parameter) is diagnosed here, and recovers by
building an
error binding that still introduces the name so that later uses of it do
not
  produce cascading errors.

The removed `:!` syntax is now rejected as an ordinary parse error.

The `form`/`:?`/`->?` ("extended types") portion of proposal #7254 is
left for a
separate change.

Assisted-by: Claude Code

toolchain/check/testdata/function/generic/resolve_used.carbon

@@ -16,7 +16,7 @@
 
 library "[[@TEST_NAME]]";
 
-fn ErrorIfNIsZero(N:! Core.IntLiteral) {
+fn ErrorIfNIsZero(generic N: Core.IntLiteral) {
   // Check that we resolve the definition of a used specific function by
   // ensuring we produce an error when doing so. Notionally this error is
   // produced as a result of instantiating the `Core.Int` template, although
@@ -124,24 +124,24 @@ fn CallNegative() {
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Core.import = import Core
 // CHECK:STDOUT:   %ErrorIfNIsZero.decl: %ErrorIfNIsZero.type = fn_decl @ErrorIfNIsZero [concrete = constants.%ErrorIfNIsZero] {
-// CHECK:STDOUT:     %N.patt.loc4_20.1: %pattern_type.dc0 = symbolic_binding_pattern N, 0 [symbolic = %N.patt.loc4_20.2 (constants.%N.patt)]
+// CHECK:STDOUT:     %N.patt.loc4_28.1: %pattern_type.dc0 = symbolic_binding_pattern N, 0 [symbolic = %N.patt.loc4_28.2 (constants.%N.patt)]
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %.loc4: type = splice_block %IntLiteral.ref [concrete = Core.IntLiteral] {
 // CHECK:STDOUT:       %.Self.frozen: %type = symbolic_binding .Self [symbolic_self = constants.%.Self.frozen]
 // CHECK:STDOUT:       %Core.ref.loc4: <namespace> = name_ref Core, imports.%Core [concrete = imports.%Core]
 // CHECK:STDOUT:       %IntLiteral.ref: type = name_ref IntLiteral, imports.%Core.IntLiteral [concrete = Core.IntLiteral]
 // CHECK:STDOUT:     }
-// CHECK:STDOUT:     %N.loc4_20.2: Core.IntLiteral = symbolic_binding N, 0 [symbolic = %N.loc4_20.1 (constants.%N)]
+// CHECK:STDOUT:     %N.loc4_28.2: Core.IntLiteral = symbolic_binding N, 0 [symbolic = %N.loc4_28.1 (constants.%N)]
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %CallNegative.decl: %CallNegative.type = fn_decl @CallNegative [concrete = constants.%CallNegative] {} {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: generic fn @ErrorIfNIsZero(%N.loc4_20.2: Core.IntLiteral) {
-// CHECK:STDOUT:   %N.patt.loc4_20.2: %pattern_type.dc0 = symbolic_binding_pattern N, 0 [symbolic = %N.patt.loc4_20.2 (constants.%N.patt)]
-// CHECK:STDOUT:   %N.loc4_20.1: Core.IntLiteral = symbolic_binding N, 0 [symbolic = %N.loc4_20.1 (constants.%N)]
+// CHECK:STDOUT: generic fn @ErrorIfNIsZero(%N.loc4_28.2: Core.IntLiteral) {
+// CHECK:STDOUT:   %N.patt.loc4_28.2: %pattern_type.dc0 = symbolic_binding_pattern N, 0 [symbolic = %N.patt.loc4_28.2 (constants.%N.patt)]
+// CHECK:STDOUT:   %N.loc4_28.1: Core.IntLiteral = symbolic_binding N, 0 [symbolic = %N.loc4_28.1 (constants.%N)]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !definition:
-// CHECK:STDOUT:   %Int.loc9_27.2: type = class_type @Int, @Int(%N.loc4_20.1) [symbolic = %Int.loc9_27.2 (constants.%Int)]
+// CHECK:STDOUT:   %Int.loc9_27.2: type = class_type @Int, @Int(%N.loc4_28.1) [symbolic = %Int.loc9_27.2 (constants.%Int)]
 // CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %Int.loc9_27.2 [symbolic = %require_complete (constants.%require_complete.4df)]
 // CHECK:STDOUT:   %pattern_type: type = pattern_type %Int.loc9_27.2 [symbolic = %pattern_type (constants.%pattern_type.142)]
 // CHECK:STDOUT:   %v.patt.loc9_15.2: @ErrorIfNIsZero.%pattern_type (%pattern_type.142) = ref_binding_pattern v [symbolic = %v.patt.loc9_15.2 (constants.%v.patt.834)]
@@ -174,7 +174,7 @@ fn CallNegative() {
 // CHECK:STDOUT:     %.loc9_27: type = splice_block %Int.loc9_27.1 [symbolic = %Int.loc9_27.2 (constants.%Int)] {
 // CHECK:STDOUT:       %Core.ref.loc9: <namespace> = name_ref Core, imports.%Core [concrete = imports.%Core]
 // CHECK:STDOUT:       %Int.ref: %Int.type = name_ref Int, imports.%Core.Int [concrete = constants.%Int.generic]
-// CHECK:STDOUT:       %N.ref: Core.IntLiteral = name_ref N, %N.loc4_20.2 [symbolic = %N.loc4_20.1 (constants.%N)]
+// CHECK:STDOUT:       %N.ref: Core.IntLiteral = name_ref N, %N.loc4_28.2 [symbolic = %N.loc4_28.1 (constants.%N)]
 // CHECK:STDOUT:       %Int.loc9_27.1: type = class_type @Int, @Int(constants.%N) [symbolic = %Int.loc9_27.2 (constants.%Int)]
 // CHECK:STDOUT:     }
 // CHECK:STDOUT:     %v: ref @ErrorIfNIsZero.%Int.loc9_27.2 (%Int) = wrapper_binding v, %v.var
@@ -216,13 +216,13 @@ fn CallNegative() {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: specific @ErrorIfNIsZero(constants.%N) {
-// CHECK:STDOUT:   %N.patt.loc4_20.2 => constants.%N.patt
-// CHECK:STDOUT:   %N.loc4_20.1 => constants.%N
+// CHECK:STDOUT:   %N.patt.loc4_28.2 => constants.%N.patt
+// CHECK:STDOUT:   %N.loc4_28.1 => constants.%N
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: specific @ErrorIfNIsZero(constants.%int_0) {
-// CHECK:STDOUT:   %N.patt.loc4_20.2 => constants.%N.patt
-// CHECK:STDOUT:   %N.loc4_20.1 => constants.%int_0
+// CHECK:STDOUT:   %N.patt.loc4_28.2 => constants.%N.patt
+// CHECK:STDOUT:   %N.loc4_28.1 => constants.%int_0
 // CHECK:STDOUT:
 // CHECK:STDOUT: !definition:
 // CHECK:STDOUT:   %Int.loc9_27.2 => constants.%i0