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/lower/testdata/class/generic.carbon

@@ -14,16 +14,16 @@
 
 package Classes;
 
-base class Base(T:! type) {
+base class Base(T: type) {
   var b: T;
 }
 
-class Derived(T:! type) {
+class Derived(T: type) {
   extend base: Base(T);
   var d: T;
 }
 
-class Adapter(T:! type) {
+class Adapter(T: type) {
   extend adapt Derived(T);
 }
 
@@ -45,12 +45,12 @@ fn CreateAdapter() -> Classes.Adapter(i32) {
 
 library "[[@TEST_NAME]]";
 
-class A(T:! type) {
+class A(T: type) {
   var x: T;
   var y: T;
 }
 
-fn Make[T:! Core.Copy](x: T, y: T) -> A(T) {
+fn Make[T: Core.Copy](x: T, y: T) -> A(T) {
   return {.x = x, .y = y};
 }
 
@@ -71,7 +71,7 @@ fn Tuples() -> A((i32, i32)) {
 
 library "[[@TEST_NAME]]";
 
-class C(T:! Core.Copy) {
+class C(T: Core.Copy) {
   fn GetBool(self) -> bool {
     return self.v;
   }

toolchain/lower/testdata/class/import.carbon

@@ -13,7 +13,7 @@
 // --- a.carbon
 
 library "X";
-base class D[T:! type](X:! T) {
+base class D[T: type](X: T) {
 }
 class C {
   fn F();

toolchain/lower/testdata/class/virtual.carbon

@@ -142,7 +142,7 @@ fn Use() {
 
 library "[[@TEST_NAME]]";
 
-base class Base(T:! type) {
+base class Base(T: type) {
   virtual fn F(unused self) { }
 }
 
@@ -150,7 +150,7 @@ base class Base(T:! type) {
 
 library "[[@TEST_NAME]]";
 
-base class Base(T:! Core.Destroy) {
+base class Base(T: Core.Destroy) {
   virtual fn F(unused self) {
     var _: T;
   }
@@ -170,7 +170,7 @@ fn F() {
 
 library "[[@TEST_NAME]]";
 
-base class Base(T:! type) {
+base class Base(T: type) {
   virtual fn F(unused self) { }
 }