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/class/fail_var_name.carbon

@@ -55,11 +55,11 @@ class C {
   // CHECK:STDERR:
   var name name: C;
 
-  // CHECK:STDERR: fail_var_name.carbon:[[@LINE+4]]:15: error: found `:!` pattern inside `var` pattern [NonRegularBindingInVarDecl]
-  // CHECK:STDERR:   var name:! C;
-  // CHECK:STDERR:               ^
+  // CHECK:STDERR: fail_var_name.carbon:[[@LINE+4]]:22: error: found generic binding inside `var` pattern [NonRegularBindingInVarDecl]
+  // CHECK:STDERR:   var generic name: C;
+  // CHECK:STDERR:                      ^
   // CHECK:STDERR:
-  var name:! C;
+  var generic name: C;
 }
 
 // CHECK:STDOUT: - filename: fail_var_name.carbon
@@ -112,9 +112,9 @@ class C {
 // CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', has_error: yes, subtree_size: 4},
 // CHECK:STDOUT:         {kind: 'VariableIntroducer', text: 'var'},
 // CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeSignature', text: 'name'},
-// CHECK:STDOUT:             {kind: 'CompileTimeBindingPatternStart', text: ':!', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'CompileTimeBindingPatternStart', text: ':', subtree_size: 2},
 // CHECK:STDOUT:             {kind: 'IdentifierNameExpr', text: 'C'},
-// CHECK:STDOUT:           {kind: 'VarBindingPattern', text: ':!', has_error: yes, subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'VarBindingPattern', text: ':', has_error: yes, subtree_size: 4},
 // CHECK:STDOUT:         {kind: 'VariablePattern', text: 'var', has_error: yes, subtree_size: 5},
 // CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', subtree_size: 7},
 // CHECK:STDOUT:     {kind: 'ClassDefinition', text: '}', subtree_size: 53},