PR c++/91369 - Implement P0784R7: constexpr new
	* cp-tree.h (CALL_FROM_NEW_OR_DELETE_P): Define.
	* init.c (build_new_1, build_vec_delete_1, build_delete): Set
	CALL_FROM_NEW_OR_DELETE_P on the CALL_EXPR to allocator functions.
	* constexpr.c (is_std_allocator_allocate): Only allow
	global replaceable allocator functions if CALL_FROM_NEW_OR_DELETE_P
	or in std::allocate<T>::{,de}allocate.
	(potential_constant_expression_1): Likewise.

	* g++.dg/cpp2a/constexpr-new6.C: New test.
	* g++.dg/cpp2a/constexpr-new7.C: New test.

From-SVN: r277732