Implement P1007R3 std::assume_aligned
	* include/std/memory (assume_aligned): Implement for C++17.
	* testsuite/20_util/assume_aligned/1.cc: New test.
	* testsuite/20_util/assume_aligned/2_neg.cc: New test.
	* testsuite/20_util/assume_aligned/3.cc: New test.

From-SVN: r266009