c++: value-dep but not type-dep decltype expr [PR105756]

c++: value-dep but not type-dep decltype expr [PR105756]

Here during ahead of time instantiation of the value-dependent but not
type-dependent decltype expression (5 % N) == 0, cp_build_binary_op folds
the operands of the == via cp_fully_fold, which performs speculative
constexpr evaluation, and from which we crash for (5 % N) due to the
value-dependence.

Since the operand folding performed by cp_build_binary_op appears to
be solely for sake of diagnosing overflow, and since these diagnostics
are suppressed when in an unevaluated context, this patch avoids this
crash by suppressing cp_build_binary_op's operand folding accordingly.

	PR c++/105756

gcc/cp/ChangeLog:

	* typeck.cc (cp_build_binary_op): Don't fold operands
	when c_inhibit_evaluation_warnings.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/decltype82.C: New test.

(cherry picked from commit 0ecb6b906f215ec56df1a555139abe9ad95414fb)