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/parse/testdata/let/fail_missing_name.carbon

@@ -18,11 +18,11 @@ let : i32 = 4;
 
 // --- fail_compiletime_binding.carbon
 
-// CHECK:STDERR: fail_compiletime_binding.carbon:[[@LINE+4]]:5: error: expected pattern [ExpectedPattern]
-// CHECK:STDERR: let :! bool = true;
-// CHECK:STDERR:     ^~
+// CHECK:STDERR: fail_compiletime_binding.carbon:[[@LINE+4]]:13: error: expected name in binding pattern [ExpectedBindingPattern]
+// CHECK:STDERR: let generic : bool = true;
+// CHECK:STDERR:             ^
 // CHECK:STDERR:
-let :! bool = true;
+let generic : bool = true;
 
 // CHECK:STDOUT: - filename: fail_runtime_binding.carbon
 // CHECK:STDOUT:   parse_tree: [
@@ -38,9 +38,12 @@ let :! bool = true;
 // CHECK:STDOUT:   parse_tree: [
 // CHECK:STDOUT:     {kind: 'FileStart', text: ''},
 // CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ':!', has_error: yes},
+// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeSignature', text: ':', has_error: yes},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPatternStart', text: ':', has_error: yes, subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'BoolTypeLiteral', text: 'bool'},
+// CHECK:STDOUT:       {kind: 'CompileTimeBindingPattern', text: ':', has_error: yes, subtree_size: 4},
 // CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
 // CHECK:STDOUT:       {kind: 'BoolLiteralTrue', text: 'true'},
-// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 5},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 8},
 // CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT:   ]