c++: 'mutable' subobject of constexpr variable [PR109745]

r13-2701-g7107ea6fb933f1 made us correctly accept during constexpr
evaluation 'mutable' member accesses of objects constructed during
that evaluation, while continuing to reject such accesses for constexpr
objects constructed outside of that evaluation, by considering the
CONSTRUCTOR_MUTABLE_POISON flag during cxx_eval_component_reference.

However, this flag is set only for the outermost CONSTRUCTOR of a
constexpr variable initializer, so if we're accessing a 'mutable' member
of a nested CONSTRUCTOR, the flag won't be set and we won't reject the
access.  This can lead to us accepting invalid code, as in the first
testcase, or even wrong code generation due to our speculative constexpr
evaluation, as in the second and third testcase.

This patch fixes this by setting CONSTRUCTOR_MUTABLE_POISON recursively
rather than only on the outermost CONSTRUCTOR.

	PR c++/109745

gcc/cp/ChangeLog:

	* typeck2.cc (poison_mutable_constructors): Define.
	(store_init_value): Use it instead of setting
	CONSTRUCTOR_MUTABLE_POISON directly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-mutable4.C: New test.
	* g++.dg/cpp0x/constexpr-mutable5.C: New test.
	* g++.dg/cpp1y/constexpr-mutable2.C: New test.