c++: 'mutable' member within constexpr [PR92505]

c++: 'mutable' member within constexpr [PR92505]

This patch permits accessing 'mutable' members of local objects during
constexpr evaluation, while continuing to reject it for global objects
(as in the last line of cpp0x/constexpr-mutable1.C).  To distinguish
between the two cases, it looks like it suffices to just check
CONSTRUCTOR_MUTABLE_POSION in cxx_eval_component_reference before
deciding to reject a DECL_MUTABLE_P member access.

	PR c++/92505

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_component_reference): Check non_constant_p
	sooner.  In C++14 or later, reject a DECL_MUTABLE_P member access
	only if CONSTRUCTOR_MUTABLE_POISION is also set.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-mutable3.C: New test.
	* g++.dg/cpp1y/constexpr-mutable1.C: New test.