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/deduce.cpp

@@ -312,7 +312,7 @@ auto DeductionContext::Deduce() -> bool {
       param_type_id =
           SemIR::ExtractScrutineeType(context().sem_ir(), param_type_id);
     } else if (context().types().IsFacetType(param_type_id)) {
-      // Given `fn F[G:! Interface](g: G)`, the type of `g` is `G as type`. For
+      // Given `fn F[G: Interface](g: G)`, the type of `g` is `G as type`. For
       // deduction, we want to ignore the `as type`, and check that the argument
       // can convert to the FacetType of the canonical facet value.
       param_id = GetCanonicalFacetOrTypeValue(context(), param_id);
@@ -361,9 +361,9 @@ auto DeductionContext::Deduce() -> bool {
     CARBON_KIND_SWITCH(param_inst) {
       // Deducing a symbolic binding pattern from an argument deduces the
       // binding as having that constant value. For example, deducing
-      // `(T:! type)` against `(i32)` deduces `T` to be `i32`. This only arises
-      // when initializing a generic parameter from an explicitly specified
-      // argument, and in this case, the argument is required to be a
+      // `(generic T: type)` against `(i32)` deduces `T` to be `i32`. This only
+      // arises when initializing a generic parameter from an explicitly
+      // specified argument, and in this case, the argument is required to be a
       // compile-time constant.
       case CARBON_KIND(SemIR::SymbolicBindingPattern bind): {
         auto& entity_name = context().entity_names().Get(bind.entity_name_id);
@@ -403,7 +403,7 @@ auto DeductionContext::Deduce() -> bool {
 
       // Deducing a symbolic binding appearing within an expression against a
       // constant value deduces the binding as having that value. For example,
-      // deducing `[T:! type](x: T)` against `("foo")` deduces `T` as `String`.
+      // deducing `[T: type](x: T)` against `("foo")` deduces `T` as `String`.
       case CARBON_KIND(SemIR::SymbolicBinding bind): {
         auto& entity_name = context().entity_names().Get(bind.entity_name_id);
         auto index = entity_name.bind_index();