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/var/fail_in_interface.carbon

@@ -22,16 +22,16 @@ interface I {
   var b: ();
 
   // CHECK:STDERR: fail_in_interface.carbon:[[@LINE+4]]:7: error: expected identifier in associated constant declaration [ExpectedAssociatedConstantIdentifier]
-  // CHECK:STDERR:   let var c:! ();
+  // CHECK:STDERR:   let var c: ();
   // CHECK:STDERR:       ^~~
   // CHECK:STDERR:
-  let var c:! ();
+  let var c: ();
 
   // CHECK:STDERR: fail_in_interface.carbon:[[@LINE+4]]:3: error: unrecognized declaration introducer [UnrecognizedDecl]
-  // CHECK:STDERR:   var d:! ();
+  // CHECK:STDERR:   var d: ();
   // CHECK:STDERR:   ^~~
   // CHECK:STDERR:
-  var d:! ();
+  var d: ();
 }
 
 // CHECK:STDOUT: - filename: fail_in_interface.carbon

toolchain/parse/testdata/var/unused.carbon

@@ -56,7 +56,7 @@ fn F(x: (), unused var y: ());
 
 // --- fail_ref_unused.carbon
 
-// CHECK:STDERR: fail_ref_unused.carbon:[[@LINE+4]]:10: error: expected name in binding pattern [ExpectedBindingPattern]
+// CHECK:STDERR: fail_ref_unused.carbon:[[@LINE+4]]:10: error: `unused` must be written before `ref` [UnusedAfterBindingModifier]
 // CHECK:STDERR: fn F(ref unused y: i32);
 // CHECK:STDERR:          ^~~~~~
 // CHECK:STDERR:
@@ -426,11 +426,13 @@ var unused (unused y: i32) = (0,);
 // CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
 // CHECK:STDOUT:       {kind: 'IdentifierNameMaybeBeforeSignature', text: 'F'},
 // CHECK:STDOUT:         {kind: 'ExplicitParamListStart', text: '('},
-// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeSignature', text: 'unused', has_error: yes},
-// CHECK:STDOUT:           {kind: 'InvalidParse', text: 'unused', has_error: yes},
-// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: 'ref', has_error: yes, subtree_size: 3},
-// CHECK:STDOUT:       {kind: 'ExplicitParamList', text: ')', has_error: yes, subtree_size: 5},
-// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 8},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeSignature', text: 'y'},
+// CHECK:STDOUT:             {kind: 'RefBindingName', text: 'ref', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:           {kind: 'LetBindingPattern', text: ':', has_error: yes, subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'ExplicitParamList', text: ')', has_error: yes, subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 10},
 // CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT:   ]
 // CHECK:STDOUT: - filename: fail_nested.carbon