tree-optimization/106868 - bogus -Wdangling-pointer diagnostic

tree-optimization/106868 - bogus -Wdangling-pointer diagnostic

The testcase shows we mishandle the case where there's a pass-through
of a pointer through a function like memcpy.  The following adjusts
handling of this copy case to require a taken address and adjust
the PHI case similarly.

	PR tree-optimization/106868
	* gimple-ssa-warn-access.cc (pass_waccess::gimple_call_return_arg_ref):
	Inline into single user ...
	(pass_waccess::check_dangling_uses): ... here and adjust the
	call and the PHI case to require that ref.aref is the address
	of the decl.

	* gcc.dg/Wdangling-pointer-pr106868.c: New testcase.