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/lower/testdata/function/generic/call_recursive_sccs_deep.carbon

@@ -11,12 +11,12 @@
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/function/generic/call_recursive_sccs_deep.carbon
 
 import Core library "io";
-fn A[T:! Core.Copy](x: T, count: i32) -> T;
-fn B[T:! Core.Copy](x: T, count: i32);
-fn C[T:! Core.Copy](x: T, count: i32);
-fn D[T:! Core.Copy](x: T, count: i32) -> T;
-fn E[T:! Core.Copy](x: T, count: i32) -> T;
-fn F[T:! Core.Copy](x: T, count: i32) -> T;
+fn A[T: Core.Copy](x: T, count: i32) -> T;
+fn B[T: Core.Copy](x: T, count: i32);
+fn C[T: Core.Copy](x: T, count: i32);
+fn D[T: Core.Copy](x: T, count: i32) -> T;
+fn E[T: Core.Copy](x: T, count: i32) -> T;
+fn F[T: Core.Copy](x: T, count: i32) -> T;
 
 // Builds on `call_recursive_mutual.carbon` and `call_recursive_diamond.carbon`
 // B-C form a mutually recursive SCC (strongly connected component), D-E-F-G
@@ -28,23 +28,23 @@ fn F[T:! Core.Copy](x: T, count: i32) -> T;
 // optimization here, if the function fingerprint may infer deduplication
 // when not including the two different generic_ids.
 
-fn A[T:! Core.Copy](x: T, count: i32) -> T {
+fn A[T: Core.Copy](x: T, count: i32) -> T {
   B(x, count);
   return D(x, count);
 }
 
-fn B[T:! Core.Copy](x: T, count: i32) {
+fn B[T: Core.Copy](x: T, count: i32) {
   C(x, count);
 }
 
-fn C[T:! Core.Copy](x: T, count: i32) {
+fn C[T: Core.Copy](x: T, count: i32) {
   if (count <= 2) {
     Core.Print(count);
     B(x, count + 1);
   }
 }
 
-fn D[T:! Core.Copy](x: T, count: i32) -> T {
+fn D[T: Core.Copy](x: T, count: i32) -> T {
   if (count > 4) {
     return x;
   }
@@ -55,17 +55,17 @@ fn D[T:! Core.Copy](x: T, count: i32) -> T {
   }
 }
 
-fn G[T:! Core.Copy](x: T, count: i32) -> T;
+fn G[T: Core.Copy](x: T, count: i32) -> T;
 
-fn E[T:! Core.Copy](x: T, count: i32) -> T {
+fn E[T: Core.Copy](x: T, count: i32) -> T {
   return G(x, count);
 }
 
-fn F[T:! Core.Copy](x: T, count: i32) -> T {
+fn F[T: Core.Copy](x: T, count: i32) -> T {
   return G(x, count);
 }
 
-fn G[T:! Core.Copy](x: T, count: i32) -> T {
+fn G[T: Core.Copy](x: T, count: i32) -> T {
   return D(x, count + 1);
 }