c++: constexpr folding in unevaluated context [PR105931]

c++: constexpr folding in unevaluated context [PR105931]

Changing the type of N from int to unsigned in decltype82.C (from
r13-986-g0ecb6b906f215e) reveals another spot where we perform constexpr
evaluation in an unevaluated context for sake of warnings, this time
from the call to shorten_compare in cp_build_binary_op, which calls
fold_for_warn.

We could (and probably should) suppress the shorten_compare warnings
when in an unevaluated context, but there's probably other callers of
fold_for_warn that are similarly affected.  So this patch takes the
approach of directly suppressing fold_for_warn when in an unevaluated
context.

	PR c++/105931

gcc/cp/ChangeLog:

	* expr.cc (fold_for_warn): Don't fold when in an unevaluated
	context.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/decltype82a.C: New test.