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/check/testdata/interop/cpp/function/export/function.carbon

@@ -17,8 +17,8 @@ fn F1() {}
 fn F2() -> i32 { return 0; }
 fn F3(_: i32) {}
 
-fn HasGenericArg(T:! type, a: T) { a; }
-fn HasDeducedArg[T:! type](a: T) { a; }
+fn HasGenericArg(generic T: type, a: T) { a; }
+fn HasDeducedArg[T: type](a: T) { a; }
 
 // --- function.carbon
 
@@ -87,12 +87,12 @@ library "[[@TEST_NAME]]";
 
 // CHECK:STDERR: fail_todo_generic.carbon:[[@LINE+9]]:1: in import [InImport]
 // CHECK:STDERR: other.carbon:7:1: error: 1 argument passed to function expecting 2 arguments [CallArgCountMismatch]
-// CHECK:STDERR: fn HasGenericArg(T:! type, a: T) { a; }
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fn HasGenericArg(generic T: type, a: T) { a; }
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // CHECK:STDERR: fail_todo_generic.carbon:[[@LINE+5]]:1: in import [InImport]
 // CHECK:STDERR: other.carbon:7:1: note: calling function declared here [InCallToEntity]
-// CHECK:STDERR: fn HasGenericArg(T:! type, a: T) { a; }
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fn HasGenericArg(generic T: type, a: T) { a; }
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // CHECK:STDERR:
 import Other;
 import Cpp inline '''