c++: Fix pretty printing of TYPENAME_TYPEs

c++: Fix pretty printing of TYPENAME_TYPEs

I noticed that in some concepts diagnostic messages, we were printing typename
types incorrectly, e.g. printing remove_reference_t<T> as

  typename remove_reference<T>::remove_reference_t

instead of

  typename remove_reference<T>::type.

Fix this by printing the TYPENAME_TYPE_FULLNAME instead of the TYPE_NAME in
cxx_pretty_printer::simple_type_specifier, which is consistent with how
dump_typename in error.c does it.

gcc/cp/ChangeLog:

	* cxx-pretty-print.c (cxx_pretty_printer::simple_type_specifier)
	[TYPENAME_TYPE]: Print the TYPENAME_TYPE_FULLNAME instead of the
	TYPE_NAME.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic4.C: New test.