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

utils/textmate/Samples/choices.carbon

@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-choice SomeChoice(T:! type) {
+choice SomeChoice(T: type) {
     Val1,
     Val2(a: i32, b: array(T, 5), t: T),
     Val3

utils/textmate/Samples/comments.carbon

@@ -4,7 +4,7 @@
 
 // This is a comment.
     //    Comment with whitespace.
-// class C; fn F[T:! A](a: T) -> i32 { return 86; }
+// class C; fn F[T: A](a: T) -> i32 { return 86; }
 //@dump-sem-ir-begin
 //@dump-sem-ir-end
 //@dump-sem-ir-

utils/textmate/Samples/functions_variables.carbon

@@ -5,14 +5,14 @@
 fn F;
 fn F1();
 fn Func(a: i32, b: A);
-fn Generic[T:! type, U:! I](a: T, b: U) -> U;
+fn Generic[T: type, U: I](a: T, b: U) -> U;
 fn Lambda(a: i32) -> i32 => return a + 5;
 fn
 
     MultiLine[
-        A:! type,
-        B:! C1,
-        C:! C2
+        A: type,
+        B: C1,
+        C: C2
     ](a: A(B, C),
       b: B,
       c: i32);
@@ -23,17 +23,17 @@ a: A;
 a: A = (b as A);
 a: i32;
 a: array(A, 5);
-a:! type;
-a:! T = b;
+generic a: type;
+generic a: T = b;
 a: A in some_list;
 var a: T;
 let b: T;
 binding: T;
-binding:! T;
+generic binding: T;
 var
     multiline: T;
 
-fn F(ref a: A, const ref b:! B);
+fn F(ref a: A, const ref b: B);
 fn F() -> T;
 fn F() -> T.U { return val; }
 fn F() -> i32 => return val;

utils/textmate/Samples/main.carbon

@@ -6,7 +6,7 @@
 
 package Carbon api;
 
-interface HasValueParam(T:! type, V:! T) {
+interface HasValueParam(T: type, V: T) {
   fn Go(self) -> T;
 }
 

utils/textmate/Samples/types.carbon

@@ -3,14 +3,14 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 class C1;
-class C2(T1:! type, T2:! C1) {}
+class C2(T1: type, T2: C1) {}
 class C3 {
     extend base: C1;
 }
 class
         (C4(
-            (T1:! type),
-            T2:!
+            (T1: type),
+            T2:
                     (A.(B(E, F(G)).C)
                                         .D))) ;
 

utils/textmate/Syntaxes/carbon.tmLanguage

@@ -607,7 +607,7 @@ THIS FILE IS AUTOGENERATED FROM carbon.tmLanguage.json. DO NOT EDIT.
 				</dict>
 				<dict>
 					<key>match</key>
-					<string>\b(and|as|impls|in|like|not|or|partial|ref|template|where)\b</string>
+					<string>\b(and|as|generic|impls|in|like|not|or|partial|ref|runtime|template|where)\b</string>
 					<key>name</key>
 					<string>keyword.control.carbon</string>
 				</dict>
@@ -797,7 +797,7 @@ THIS FILE IS AUTOGENERATED FROM carbon.tmLanguage.json. DO NOT EDIT.
 				</dict>
 				<dict>
 					<key>begin</key>
-					<string>:!?</string>
+					<string>:</string>
 					<key>comment</key>
 					<string>Matches types in bindings.</string>
 					<key>end</key>

utils/vscode/carbon.tmLanguage.json

@@ -226,7 +226,7 @@
         },
         {
           "comment": "Matches types in bindings.",
-          "begin": ":!?",
+          "begin": ":",
           "end": "(?=([;{=]|->|in|where))",
           "patterns": [
             { "include": "#common" },
@@ -346,7 +346,7 @@
         },
         {
           "name": "keyword.control.carbon",
-          "match": "\\b(and|as|impls|in|like|not|or|partial|ref|template|where)\\b"
+          "match": "\\b(and|as|generic|impls|in|like|not|or|partial|ref|runtime|template|where)\\b"
         }
       ]
     },