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/generic/stringify.carbon

@@ -33,8 +33,8 @@ var w: EmptyParams() = v;
 
 library "[[@TEST_NAME]]";
 
-class Outer(T:! type) {
-  class Inner(U:! type) {
+class Outer(T: type) {
+  class Inner(U: type) {
   }
 }
 
@@ -54,7 +54,7 @@ var w: Outer({}*).Inner({.a: i32}*) = v;
 
 library "[[@TEST_NAME]]";
 
-class C(N:! i32) {}
+class C(N: i32) {}
 
 // CHECK:STDERR: fail_int_value.carbon:[[@LINE+7]]:1: error: cannot implicitly convert expression of type `()` to `C(123)` [ConversionFailure]
 // CHECK:STDERR: var v: C(123) = ();
@@ -74,7 +74,7 @@ class D {
   var b: i32;
 }
 
-class E(F:! D) {}
+class E(F: D) {}
 
 // CHECK:STDERR: fail_class_param.carbon:[[@LINE+7]]:1: error: cannot implicitly convert expression of type `E({.a = 3, .b = 4})` to `E({.a = 1, .b = 2})` [ConversionFailure]
 // CHECK:STDERR: var g: E({.a = 1, .b = 2}) = {} as E({.a = 3, .b = 4} as D);
@@ -264,9 +264,9 @@ var g: E({.a = 1, .b = 2}) = {} as E({.a = 3, .b = 4} as D);
 // CHECK:STDOUT:   %Outer.decl: %Outer.type = class_decl @Outer [concrete = constants.%Outer.generic] {
 // CHECK:STDOUT:     %T.patt.loc4_14.1: %pattern_type.98f = symbolic_binding_pattern T, 0 [symbolic = %T.patt.loc4_14.2 (constants.%T.patt.d47)]
 // CHECK:STDOUT:   } {
-// CHECK:STDOUT:     %.loc4_17.1: type = splice_block %.loc4_17.2 [concrete = type] {
+// CHECK:STDOUT:     %.loc4_16.1: type = splice_block %.loc4_16.2 [concrete = type] {
 // CHECK:STDOUT:       %.Self.frozen: %type = symbolic_binding .Self [symbolic_self = constants.%.Self.frozen]
-// CHECK:STDOUT:       %.loc4_17.2: type = type_literal type [concrete = type]
+// CHECK:STDOUT:       %.loc4_16.2: type = type_literal type [concrete = type]
 // CHECK:STDOUT:     }
 // CHECK:STDOUT:     %T.loc4_14.2: type = symbolic_binding T, 0 [symbolic = %T.loc4_14.1 (constants.%T.67d)]
 // CHECK:STDOUT:   }
@@ -316,9 +316,9 @@ var g: E({.a = 1, .b = 2}) = {} as E({.a = 3, .b = 4} as D);
 // CHECK:STDOUT:     %Inner.decl: @Outer.%Inner.type (%Inner.type.8f7) = class_decl @Inner [symbolic = @Outer.%Inner.generic (constants.%Inner.generic.b3a)] {
 // CHECK:STDOUT:       %U.patt.loc5_16.1: %pattern_type.98f = symbolic_binding_pattern U, 1 [symbolic = %U.patt.loc5_16.2 (constants.%U.patt)]
 // CHECK:STDOUT:     } {
-// CHECK:STDOUT:       %.loc5_19.1: type = splice_block %.loc5_19.2 [concrete = type] {
+// CHECK:STDOUT:       %.loc5_18.1: type = splice_block %.loc5_18.2 [concrete = type] {
 // CHECK:STDOUT:         %.Self.frozen: %type = symbolic_binding .Self [symbolic_self = constants.%.Self.frozen]
-// CHECK:STDOUT:         %.loc5_19.2: type = type_literal type [concrete = type]
+// CHECK:STDOUT:         %.loc5_18.2: type = type_literal type [concrete = type]
 // CHECK:STDOUT:       }
 // CHECK:STDOUT:       %U.loc5_16.2: type = symbolic_binding U, 1 [symbolic = %U.loc5_16.1 (constants.%U)]
 // CHECK:STDOUT:     }