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/interop/cpp/class/export/class.carbon

@@ -78,7 +78,7 @@ library "[[@TEST_NAME]]";
 
 import Cpp;
 
-class B(N:! i32) {
+class B(N: i32) {
   var a: array(i8, N);
 }
 

toolchain/check/testdata/interop/cpp/class/import/class.carbon

@@ -232,7 +232,7 @@ import Cpp inline '''
 class Class1;
 ''';
 
-class Generic(x:! Core.Copy) {
+class Generic(x: Core.Copy) {
 }
 
 // CHECK:STDERR: fail_copy_witness_of_incomplete_class.carbon:[[@LINE+14]]:19: error: argument to C++ call has incomplete type `Cpp.Class1` [IncompleteTypeInCopyWitness]
@@ -242,8 +242,8 @@ class Generic(x:! Core.Copy) {
 // CHECK:STDERR: class Class1;
 // CHECK:STDERR:       ^
 // CHECK:STDERR: fail_copy_witness_of_incomplete_class.carbon:[[@LINE-9]]:15: note: initializing generic parameter `x` declared here [InitializingGenericParam]
-// CHECK:STDERR: class Generic(x:! Core.Copy) {
-// CHECK:STDERR:               ^~~~~~~~~~~~~
+// CHECK:STDERR: class Generic(x: Core.Copy) {
+// CHECK:STDERR:               ^~~~~~~~~~~~
 // CHECK:STDERR:
 // CHECK:STDERR: fail_copy_witness_of_incomplete_class.carbon:[[@LINE+4]]:15: warning: binding `p2` unused [UnusedBinding]
 // CHECK:STDERR: fn GenericUse(p2: Generic(Cpp.Class1)) {

toolchain/check/testdata/interop/cpp/class/import/constructor.carbon

@@ -49,7 +49,7 @@ fn F() {
   //@dump-sem-ir-end
 }
 
-fn G(T:! Core.Default & Core.Destroy) {
+fn G(generic T: Core.Default & Core.Destroy) {
   var unused x: T;
 }
 
@@ -75,7 +75,7 @@ library "[[@TEST_NAME]]";
 
 import Cpp library "default.h";
 
-fn G(T:! Core.Default & Core.Destroy) {
+fn G(generic T: Core.Default & Core.Destroy) {
   var unused x: T;
 }
 
@@ -83,9 +83,9 @@ fn CallG() {
   // CHECK:STDERR: fail_no_default.carbon:[[@LINE+7]]:3: error: cannot convert type `NoDefault` into type implementing `Core.Default & Core.Destroy` [ConversionFailureTypeToFacet]
   // CHECK:STDERR:   G(Cpp.NoDefault);
   // CHECK:STDERR:   ^~~~~~~~~~~~~~~~
-  // CHECK:STDERR: fail_no_default.carbon:[[@LINE-8]]:6: note: initializing generic parameter `T` declared here [InitializingGenericParam]
-  // CHECK:STDERR: fn G(T:! Core.Default & Core.Destroy) {
-  // CHECK:STDERR:      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_no_default.carbon:[[@LINE-8]]:14: note: initializing generic parameter `T` declared here [InitializingGenericParam]
+  // CHECK:STDERR: fn G(generic T: Core.Default & Core.Destroy) {
+  // CHECK:STDERR:              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   // CHECK:STDERR:
   G(Cpp.NoDefault);
 }