The new constexpr destructor on std::allocator breaks compilation with
Clang in C++2a mode. This only makes it constexpr if the compiler
supports the P0784R7 features.

	* include/bits/allocator.h: Check __cpp_constexpr_dynamic_alloc
	before making the std::allocator destructor constexpr.
	* testsuite/20_util/allocator/requirements/constexpr.cc: New test.

From-SVN: r277458