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/array/import.carbon

@@ -31,7 +31,7 @@ fn G(n: i32) -> i32 {
 library "[[@TEST_NAME]]";
 
 interface I {
-  let value:! array(i32, 1);
+  let value: array(i32, 1);
 }
 
 class C {}

toolchain/check/testdata/array/init_dependent_bound.carbon

@@ -14,7 +14,7 @@
 
 library "[[@TEST_NAME]]";
 
-fn G(T:! type) {
+fn G(generic T: type) {
   // We can initialize this without knowing T.
   //@dump-sem-ir-begin
   var unused arr: array(T, 0) = ();
@@ -31,7 +31,7 @@ fn H() {
 
 library "[[@TEST_NAME]]";
 
-fn F(N:! i32) {
+fn F(generic N: i32) {
   // CHECK:STDERR: fail_init_dependent_bound.carbon:[[@LINE+4]]:35: error: cannot initialize array with dependent bound from a list of initializers [ArrayInitDependentBound]
   // CHECK:STDERR:   var unused arr: array(i32, N) = (1, 2, 3);
   // CHECK:STDERR:                                   ^~~~~~~~~
@@ -44,7 +44,7 @@ fn F(N:! i32) {
 library "[[@TEST_NAME]]";
 
 // TODO: This should be valid.
-fn G(template N:! i32) {
+fn G(template N: i32) {
   //@dump-sem-ir-begin
   // CHECK:STDERR: fail_todo_init_template_dependent_bound.carbon:[[@LINE+4]]:19: error: cannot evaluate type expression [TypeExprEvaluationFailure]
   // CHECK:STDERR:   var unused arr: array(i32, N) = (1, 2, 3);
@@ -94,11 +94,11 @@ fn H() { G(3); }
 // CHECK:STDOUT:   %.a02: type = fn_type_with_self_type %Destroy.WithSelf.Op.type.ccc, %Destroy.facet.396 [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: generic fn @G(%T.loc4_7.2: type) {
+// CHECK:STDOUT: generic fn @G(%T.loc4_15.2: type) {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT:
 // CHECK:STDOUT: !definition:
-// CHECK:STDOUT:   %array_type.loc7_29.2: type = array_type constants.%int_0, %T.loc4_7.1 [symbolic = %array_type.loc7_29.2 (constants.%array_type.1b3)]
+// CHECK:STDOUT:   %array_type.loc7_29.2: type = array_type constants.%int_0, %T.loc4_15.1 [symbolic = %array_type.loc7_29.2 (constants.%array_type.1b3)]
 // CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %array_type.loc7_29.2 [symbolic = %require_complete (constants.%require_complete)]
 // CHECK:STDOUT:   %pattern_type: type = pattern_type %array_type.loc7_29.2 [symbolic = %pattern_type (constants.%pattern_type.bd6)]
 // CHECK:STDOUT:   %arr.patt.loc7_17.2: @G.%pattern_type (%pattern_type.bd6) = ref_binding_pattern arr [symbolic = %arr.patt.loc7_17.2 (constants.%arr.patt.770)]
@@ -119,7 +119,7 @@ fn H() { G(3); }
 // CHECK:STDOUT:     %.loc7_3.1: init @G.%array_type.loc7_29.2 (%array_type.1b3) = converted %.loc7_34.1, %.loc7_34.2 [symbolic = %array (constants.%array.ca4)]
 // CHECK:STDOUT:     assign %arr.var, %.loc7_3.1
 // CHECK:STDOUT:     %.loc7_29: type = splice_block %array_type.loc7_29.1 [symbolic = %array_type.loc7_29.2 (constants.%array_type.1b3)] {
-// CHECK:STDOUT:       %T.ref: type = name_ref T, %T.loc4_7.2 [symbolic = %T.loc4_7.1 (constants.%T)]
+// CHECK:STDOUT:       %T.ref: type = name_ref T, %T.loc4_15.2 [symbolic = %T.loc4_15.1 (constants.%T)]
 // CHECK:STDOUT:       %int_0: Core.IntLiteral = int_value 0 [concrete = constants.%int_0]
 // CHECK:STDOUT:       %array_type.loc7_29.1: type = array_type %int_0, %T.ref [symbolic = %array_type.loc7_29.2 (constants.%array_type.1b3)]
 // CHECK:STDOUT:     }
@@ -146,13 +146,13 @@ fn H() { G(3); }
 // CHECK:STDOUT: fn @Destroy.Op(%self.param: ref %array_type.988) = "no_op";
 // CHECK:STDOUT:
 // CHECK:STDOUT: specific @G(constants.%T) {
-// CHECK:STDOUT:   %T.patt.loc4_7.2 => constants.%T.patt
-// CHECK:STDOUT:   %T.loc4_7.1 => constants.%T
+// CHECK:STDOUT:   %T.patt.loc4_15.2 => constants.%T.patt
+// CHECK:STDOUT:   %T.loc4_15.1 => constants.%T
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: specific @G(constants.%C) {
-// CHECK:STDOUT:   %T.patt.loc4_7.2 => constants.%T.patt
-// CHECK:STDOUT:   %T.loc4_7.1 => constants.%C
+// CHECK:STDOUT:   %T.patt.loc4_15.2 => constants.%T.patt
+// CHECK:STDOUT:   %T.loc4_15.1 => constants.%C
 // CHECK:STDOUT:
 // CHECK:STDOUT: !definition:
 // CHECK:STDOUT:   %array_type.loc7_29.2 => constants.%array_type.988