c++: handle _FloatNN redeclaration like bool [PR107128]

c++: handle _FloatNN redeclaration like bool [PR107128]

It's been inconvenient to compile testcases preprocessed with GCC 12 or
earlier because they break on

  typedef __float128 _Float128;

We already had code for handling this with bool and wchar_t, it just needs
to be extended to _FloatNN as well.

	PR c++/107128

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_set_decl_spec_type): Use
	redefined_builtin_type for extended_float_type_p.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/pragma-system_header6.h: New test.
	* g++.dg/warn/pragma-system_header6.C: New test.