[Sema] Note when we encounter a problem in ExprConstant.

Currently, the constexpr evaluator is very conservative about unmodeled
side-effects when we're evaluating an expression in a mode that allows
such side-effects.

This patch makes us note when we might have actually encountered an
unmodeled side-effect, which allows us to be more accurate when we know
an unmodeled side-effect couldn't have occurred.

This patch has been split into two commits; this one primarily
introduces the bits necessary to track whether we might have potentially
hit such a side-effect. The one that actually does the tracking (which
boils down to more or less a rename of keepEvaluatingAfterFailure to
noteFailure) is coming soon.

Differential Revision: http://reviews.llvm.org/D18540

llvm-svn: 270781