libstdc++: Fix some -Wunused warnings in tests

libstdc++-v3/ChangeLog:

	* include/ext/pb_ds/detail/type_utils.hpp (PB_DS_STATIC_ASSERT):
	Add unused attribute to avoid -Wunused-local-typedef warnings.
	* testsuite/17_intro/tag_type_explicit_ctor.cc: Add pragma to
	ignore -Wunused-variable warnings