c++: Fix ICE with -Wmismatched-tags [PR93869]

c++: Fix ICE with -Wmismatched-tags [PR93869]

This is a crash in cp_parser_check_class_key:
  tree type_decl = TYPE_MAIN_DECL (type);
  tree name = DECL_NAME (type_decl); // HERE
because TYPE_MAIN_DECL of type was null as it's not a class type.
Instead of checking CLASS_TYPE_P we should simply check class_key
a bit earlier (in this case it was typename_type).

2020-02-24  Marek Polacek  <polacek@redhat.com>

	PR c++/93869 - ICE with -Wmismatched-tags.
	* parser.c (cp_parser_check_class_key): Check class_key earlier.

	* g++.dg/warn/Wmismatched-tags-2.C: New test.