Fix tests that fail in C++2a mode

Fix tests that fail in C++2a mode

The nested typedefs in std::reference_wrapper are no longer present in
C++2a mode, so skip the tests that check for them.

The addition of the [[nodiscard]] attribute to a few functions causes
some failures in tests that intentionally ignore the return value.
Casting the result to void suppresses the new warnings.

	* testsuite/20_util/reference_wrapper/result_type.cc: Disable for
	C++2a.
	* testsuite/20_util/reference_wrapper/typedefs-2.cc: Likewise.
	* testsuite/20_util/reference_wrapper/typedefs-3.cc: Likewise.
	* testsuite/20_util/reference_wrapper/typedefs.cc: Likewise.
	* testsuite/30_threads/async/54297.cc: Suppress nodiscard warning.
	* testsuite/ext/array_allocator/26875.cc: Likewise.
	* testsuite/ext/pool_allocator/allocate_chunk.cc: Likewise.
	* testsuite/util/replacement_memory_operators.h: Likewise.
	* testsuite/util/testsuite_allocator.h: Likewise.

From-SVN: r268355