c++: don't do constexpr folding in unevaluated context

c++: don't do constexpr folding in unevaluated context

The implicit constexpr patch revealed that we were doing constant evaluation
of arbitrary expressions in unevaluated contexts, leading to failure when we
tried to evaluate e.g. a call to declval.  This is wrong more generally;
only manifestly-constant-evaluated expressions should be evaluated within
an unevaluated operand.

Making this change revealed a case we were failing to mark as manifestly
constant-evaluated.

gcc/cp/ChangeLog:

	* constexpr.c (maybe_constant_value): Don't evaluate
	in an unevaluated operand unless manifestly const-evaluated.
	(fold_non_dependent_expr_template): Likewise.
	* decl.c (compute_array_index_type_loc): This context is
	manifestly constant-evaluated.