c++: Fix up ptr.~PTR () handling [PR96721]

c++: Fix up ptr.~PTR () handling [PR96721]

The following testcase is miscompiled, because build_trivial_dtor_call
handles the case when instance is a pointer by adding a clobber to what
the pointer points to (which is desirable e.g. for delete) rather than the
pointer itself.  That is I think always desirable behavior for references,
but for pointers for the pseudo dtor case it is not.

2020-08-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96721
	* cp-tree.h (build_trivial_dtor_call): Add bool argument defaulted
	to false.
	* call.c (build_trivial_dtor_call): Add NO_PTR_DEREF argument.  If
	instance is a pointer and NO_PTR_DEREF is true, clobber the pointer
	rather than what it points to.
	* semantics.c (finish_call_expr): Call build_trivial_dtor_call with
	true as NO_PTR_DEREF.

	* g++.dg/opt/flifetime-dse8.C: New test.