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/struct/import.carbon

@@ -18,7 +18,7 @@ var a_ref: {.a: i32} = {.a = 0};
 var b_ref: {.a: {.b: i32, .c: (i32,)}, .d: i32} =
     {.a = {.b = 0, .c = (0,)}, .d = 0};
 
-class C(S:! {.a: i32, .b: i32}) {}
+class C(S: {.a: i32, .b: i32}) {}
 fn F() -> C({.a = 1, .b = 2});
 
 // --- implicit.impl.carbon
@@ -39,8 +39,8 @@ impl package Implicit;
 // CHECK:STDERR:              ^~~~~~~~~~~~~~~~
 // CHECK:STDERR: fail_bad_type.impl.carbon:[[@LINE-4]]:1: in import [InImport]
 // CHECK:STDERR: implicit.carbon:8:9: note: initializing generic parameter `S` declared here [InitializingGenericParam]
-// CHECK:STDERR: class C(S:! {.a: i32, .b: i32}) {}
-// CHECK:STDERR:         ^~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: class C(S: {.a: i32, .b: i32}) {}
+// CHECK:STDERR:         ^~~~~~~~~~~~~~~~~~~~~
 // CHECK:STDERR:
 var c_bad: C({.c = 1, .d = 2}) = F();
 
@@ -61,7 +61,7 @@ var c_bad: C({.a = 3, .b = 4}) = F();
 library "[[@TEST_NAME]]";
 
 interface I {
-  let value:! {.x: i32};
+  let value: {.x: i32};
 }
 
 class C {}