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/function/generic/undefined.carbon

@@ -14,7 +14,7 @@
 
 library "[[@TEST_NAME]]";
 
-fn Defined[T:! type](unused x: T) {}
+fn Defined[T: type](unused x: T) {}
 
 fn CallDefined() {
   Defined(0 as i32);
@@ -24,27 +24,27 @@ fn CallDefined() {
 
 library "[[@TEST_NAME]]";
 
-fn Defined[T:! type](x: T);
+fn Defined[T: type](x: T);
 
 fn CallDefined() {
   Defined(0 as i32);
 }
 
-fn Defined[T:! type](unused x: T) {}
+fn Defined[T: type](unused x: T) {}
 
 // --- fail_call_undefined.carbon
 
 library "[[@TEST_NAME]]";
 
-fn Undefined[T:! type](x: T);
+fn Undefined[T: type](x: T);
 
 fn CallUndefined() {
   // CHECK:STDERR: fail_call_undefined.carbon:[[@LINE+7]]:3: error: use of undefined generic function [MissingGenericFunctionDefinition]
   // CHECK:STDERR:   Undefined(0 as i32);
   // CHECK:STDERR:   ^~~~~~~~~
   // CHECK:STDERR: fail_call_undefined.carbon:[[@LINE-6]]:1: note: generic function declared here [MissingGenericFunctionDefinitionHere]
-  // CHECK:STDERR: fn Undefined[T:! type](x: T);
-  // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fn Undefined[T: type](x: T);
+  // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
   // CHECK:STDERR:
   Undefined(0 as i32);
 }