Handle __assume in the VoidExprEvaluator

This is a follow-up to an IRC conversation with Richard last night; __assume
does not evaluate its argument, and so the argument should not contribute to
whether (__assume(e), constant) can be used where a constant is required.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213267 91177308-0d34-0410-b5e6-96231b3b80d8