PR c++/91369 - Implement P0784R7: constexpr new
	* constexpr.c (cxx_replaceable_global_alloc_fn): Don't return true
	for placement new.
	(cxx_placement_new_fn, is_std_construct_at): New functions.
	(cxx_eval_call_expression): Allow placement new in std::construct_at.
	(potential_constant_expression_1): Likewise.

	* g++.dg/cpp2a/constexpr-new5.C: New test.

From-SVN: r277649