Allow use of an elaborated type specifier in a _Generic association in C++

Currently, Clang accepts this code in C mode (where the tag is required
to be used) but rejects it in C++ mode thinking that the association is
defining a new type.

void foo(void) {
  struct S { int a; };
  _Generic(something, struct S : 1);
}
Clang thinks this in C++ because it sees struct S : when parsing the
class specifier and decides that must be a type definition (because the
colon signifies the presence of a base class type). This patch adds a
new declarator context to represent a _Generic association so that we
can distinguish these situations properly.

Fixes #55562

Differential Revision: https://reviews.llvm.org/D126969