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/class/self/fail_self_param.carbon

@@ -16,16 +16,16 @@ library "[[@TEST_NAME]]";
 // `self` is not a valid parameter of a class declaration even when placed in
 // the explicit parameter list.
 // CHECK:STDERR: fail_self_param.carbon:[[@LINE+4]]:9: error: `self` parameter only allowed on functions [SelfParameterNotAllowed]
-// CHECK:STDERR: class C(self:! type, x:! self) {}
-// CHECK:STDERR:         ^~~~~~~~~~~
+// CHECK:STDERR: class C(self: type, x: self) {}
+// CHECK:STDERR:         ^~~~~~~~~~
 // CHECK:STDERR:
-class C(self:! type, x:! self) {}
+class C(self: type, x: self) {}
 // CHECK:STDERR: fail_self_param.carbon:[[@LINE+7]]:8: error: 1 argument passed to generic class expecting 2 arguments [CallArgCountMismatch]
 // CHECK:STDERR: var v: C(());
 // CHECK:STDERR:        ^~~~~
 // CHECK:STDERR: fail_self_param.carbon:[[@LINE-4]]:1: note: calling generic class declared here [InCallToEntity]
-// CHECK:STDERR: class C(self:! type, x:! self) {}
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: class C(self: type, x: self) {}
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // CHECK:STDERR:
 var v: C(());
 
@@ -35,12 +35,12 @@ library "[[@TEST_NAME]]";
 // `self` in the implicit parameter list of a class declaration is reported as
 // both being in the wrong list and not allowed on a class at all.
 // CHECK:STDERR: fail_self_param_implicit.carbon:[[@LINE+8]]:9: error: `self` must be declared in the explicit parameter list [SelfInImplicitParamList]
-// CHECK:STDERR: class C[self:! type](x:! self) {}
-// CHECK:STDERR:         ^~~~~~~~~~~
+// CHECK:STDERR: class C[self: type](x: self) {}
+// CHECK:STDERR:         ^~~~~~~~~~
 // CHECK:STDERR:
 // CHECK:STDERR: fail_self_param_implicit.carbon:[[@LINE+4]]:9: error: `self` parameter only allowed on functions [SelfParameterNotAllowed]
-// CHECK:STDERR: class C[self:! type](x:! self) {}
-// CHECK:STDERR:         ^~~~~~~~~~~
+// CHECK:STDERR: class C[self: type](x: self) {}
+// CHECK:STDERR:         ^~~~~~~~~~
 // CHECK:STDERR:
-class C[self:! type](x:! self) {}
+class C[self: type](x: self) {}
 var v: C(());