The libstdc++ manual documents that _T can not be used, because it's a
macro in system headers on some targets.

Backport from mainline
2020-05-01  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/94901
	* include/std/type_traits (__is_complete_or_unbounded): Replace
	BADNAME _T with _Tp.
	* testsuite/17_intro/badnames.cc: New test.