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/forward_decl.carbon

@@ -12,7 +12,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/function/generic/forward_decl.carbon
 
-fn F(T:! type);
+fn F(generic T: type);
 
 // CHECK:STDOUT: --- forward_decl.carbon
 // CHECK:STDOUT:
@@ -31,25 +31,25 @@ fn F(T:! type);
 // CHECK:STDOUT:     .F = %F.decl
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
-// CHECK:STDOUT:     %T.patt.loc15_7.1: %pattern_type = symbolic_binding_pattern T, 0 [symbolic = %T.patt.loc15_7.2 (constants.%T.patt)]
+// CHECK:STDOUT:     %T.patt.loc15_15.1: %pattern_type = symbolic_binding_pattern T, 0 [symbolic = %T.patt.loc15_15.2 (constants.%T.patt)]
 // CHECK:STDOUT:   } {
-// CHECK:STDOUT:     %.loc15_10.1: type = splice_block %.loc15_10.2 [concrete = type] {
+// CHECK:STDOUT:     %.loc15_17.1: type = splice_block %.loc15_17.2 [concrete = type] {
 // CHECK:STDOUT:       %.Self.frozen: %type = symbolic_binding .Self [symbolic_self = constants.%.Self.frozen]
-// CHECK:STDOUT:       %.loc15_10.2: type = type_literal type [concrete = type]
+// CHECK:STDOUT:       %.loc15_17.2: type = type_literal type [concrete = type]
 // CHECK:STDOUT:     }
-// CHECK:STDOUT:     %T.loc15_7.2: type = symbolic_binding T, 0 [symbolic = %T.loc15_7.1 (constants.%T)]
+// CHECK:STDOUT:     %T.loc15_15.2: type = symbolic_binding T, 0 [symbolic = %T.loc15_15.1 (constants.%T)]
 // CHECK:STDOUT:   }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: generic fn @F(%T.loc15_7.2: type) {
-// CHECK:STDOUT:   %T.patt.loc15_7.2: %pattern_type = symbolic_binding_pattern T, 0 [symbolic = %T.patt.loc15_7.2 (constants.%T.patt)]
-// CHECK:STDOUT:   %T.loc15_7.1: type = symbolic_binding T, 0 [symbolic = %T.loc15_7.1 (constants.%T)]
+// CHECK:STDOUT: generic fn @F(%T.loc15_15.2: type) {
+// CHECK:STDOUT:   %T.patt.loc15_15.2: %pattern_type = symbolic_binding_pattern T, 0 [symbolic = %T.patt.loc15_15.2 (constants.%T.patt)]
+// CHECK:STDOUT:   %T.loc15_15.1: type = symbolic_binding T, 0 [symbolic = %T.loc15_15.1 (constants.%T)]
 // CHECK:STDOUT:
 // CHECK:STDOUT:   fn();
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: specific @F(constants.%T) {
-// CHECK:STDOUT:   %T.patt.loc15_7.2 => constants.%T.patt
-// CHECK:STDOUT:   %T.loc15_7.1 => constants.%T
+// CHECK:STDOUT:   %T.patt.loc15_15.2 => constants.%T.patt
+// CHECK:STDOUT:   %T.loc15_15.1 => constants.%T
 // CHECK:STDOUT: }
 // CHECK:STDOUT: