c++: Fix ICE with -Wmismatched-tags [PR105725]

c++: Fix ICE with -Wmismatched-tags [PR105725]

Here we ICE with -Wmismatched-tags on something like

  template <class T>
  bool B<T, enable_if_t<is_class_v<class T::foo>>>;

Specifically, the "class T::foo" bit.  There, class_decl_loc_t::add gets
a TYPENAME_TYPE as TYPE, rather than a class/union type, so checking
TYPE_BEING_DEFINED will crash.  I think it's OK to allow a TYPENAME_TYPE to
slip into that function; we just shouldn't consider the 'class' tag redundant
(which works as a 'typename').  In fact, every other compiler *requires* it.

	PR c++/105725

gcc/cp/ChangeLog:

	* parser.cc (class_decl_loc_t::add): Check CLASS_TYPE_P.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wmismatched-tags-10.C: New test.