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/var/import/constexpr.carbon

@@ -19,7 +19,7 @@ import Cpp inline '''
 constexpr bool b = true;
 ''';
 
-class C(B:! bool) {}
+class C(B: bool) {}
 fn F() -> C(Cpp.b);
 let x: C(true) = F();
 
@@ -31,7 +31,7 @@ import Cpp inline '''
 constexpr int i = 123;
 ''';
 
-class C(I:! i32) {}
+class C(I: i32) {}
 fn F() -> C(Cpp.i);
 let x: C(123) = F();
 
@@ -43,7 +43,7 @@ import Cpp inline '''
 constexpr float flt = 123.5;
 ''';
 
-class C(V:! f32) {}
+class C(V: f32) {}
 fn F() -> C(Cpp.flt);
 let x: C(123.5) = F();
 
@@ -56,6 +56,6 @@ int i = 123;
 constexpr int* _Nonnull ptr = &i;
 ''';
 
-class C(V:! i32*) {}
+class C(V: i32*) {}
 fn F() -> C(Cpp.ptr);
 let x: C(&Cpp.i) = F();