libstdc++: Fix testsuite failures and warnings due to is_pod deprecation

libstdc++: Fix testsuite failures and warnings due to is_pod deprecation

With -std=gnu++2a and -Wsystem-headers the std::is_pod deprecation
causes some new diagnostics. This suppresses them.

	* include/experimental/type_traits (experimental::is_pod_v): Disable
	-Wdeprecated-declarations warnings around reference to std::is_pod.
	* include/std/type_traits (is_pod_v): Likewise.
	* testsuite/18_support/max_align_t/requirements/2.cc: Also check
	is_standard_layout and is_trivial. Do not check is_pod for C++20.
	* testsuite/20_util/is_pod/requirements/explicit_instantiation.cc:
	Add -Wno-deprecated for C++20.
	* testsuite/20_util/is_pod/requirements/typedefs.cc: Likewise.
	* testsuite/20_util/is_pod/value.cc: Likewise.
	* testsuite/experimental/type_traits/value.cc: Likewise.

From-SVN: r280066