Superficial support for template modifier on symbolic bindings. (#4948)

Change parse tree from `template (T:! type)` to `(template T):! type`,
so that we have information about whether a binding is a template
binding available when forming the representation of the binding
pattern. This incidentally fixes a bug that we would accept `template
addr A:! B` instead of the intended `addr template A:! B`.

Track whether a symbolic binding is a template binding on the
`EntityName` object. I'm borrowing a bit from the `CompileTimeBindIndex`
for this in order to avoid making `EntityName`s larger. Longer-term, we
should think about using a different representation for symbolic
bindings, to avoid including these fields in all `EntityName`s, but
that's out of scope for this change.

So far, template bindings are treated as having the same phase as
checked bindings, but that will change in a future PR.

toolchain/check/handle_interface.cpp

@@ -169,10 +169,10 @@ auto HandleParseNode(Context& context,
   // We model `Self` as a symbolic binding whose type is the interface.
   // Because there is no equivalent non-symbolic value, we use `None` as
   // the `value_id` on the `BindSymbolicName`.
-  auto entity_name_id = context.entity_names().Add(
-      {.name_id = SemIR::NameId::SelfType,
-       .parent_scope_id = interface_info.scope_id,
-       .bind_index = context.scope_stack().AddCompileTimeBinding()});
+  auto entity_name_id = context.entity_names().AddSymbolicBindingName(
+      SemIR::NameId::SelfType, interface_info.scope_id,
+      context.scope_stack().AddCompileTimeBinding(),
+      /*is_template=*/false);
   interface_info.self_param_id =
       context.AddInst(SemIR::LocIdAndInst::NoLoc<SemIR::BindSymbolicName>(
           {.type_id = self_type_id,