libstdc++/69406 Fix test to check for supported headers

libstdc++/69406 Fix test to check for supported headers

	PR libstdc++/69406
	* include/bits/cpp_type_traits.h: Ensure C++ language linkage.
	* include/ext/type_traits.h: Likewise.
	* testsuite/17_intro/headers/c++2011/linkage.cc: Check autoconf macros
	for presence of C headers.
	* testsuite/ext/type_traits/add_unsigned_floating_neg.cc: Adjust
	dg-error line number.
	* testsuite/ext/type_traits/add_unsigned_integer_neg.cc: Likewise.
	* testsuite/ext/type_traits/remove_unsigned_floating_neg.cc: Likewise.
	* testsuite/ext/type_traits/remove_unsigned_integer_neg.cc: Likewise.

From-SVN: r232672