Teach compute_objsize about placement new [PR100876].

Resolves:
PR c++/100876 - -Wmismatched-new-delete should understand placement new when it's not inlined

gcc/ChangeLog:

	PR c++/100876
	* builtins.c (gimple_call_return_array): Check for attribute fn spec.
	Handle calls to placement new.
	(ndecl_dealloc_argno): Avoid placement delete.

gcc/testsuite/ChangeLog:

	PR c++/100876
	* g++.dg/warn/Wmismatched-new-delete-4.C: New test.
	* g++.dg/warn/Wmismatched-new-delete-5.C: New test.
	* g++.dg/warn/Wstringop-overflow-7.C: New test.
	* g++.dg/warn/Wfree-nonheap-object-6.C: New test.
	* g++.dg/analyzer/placement-new.C: Prune out expected warning.