PR c++/91548 - fix detecting modifying const objects for ARRAY_REF.

PR c++/91548 - fix detecting modifying const objects for ARRAY_REF.

This fixes a bogus "modifying a const object" error for an array that actually
isn't declared const.  The problem was how I handled ARRAY_REFs here; we
shouldn't look at the ARRAY_REF itself, but at the array its accessing.

	* constexpr.c (cxx_eval_store_expression): Don't call
	modifying_const_object_p for ARRAY_REF.

	* g++.dg/cpp1y/constexpr-tracking-const15.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const16.C: New test.
	* g++.dg/cpp1z/constexpr-tracking-const1.C: New test.

From-SVN: r277591