fix rdar://9289603 - clang should fold trivial ?: for enums as well as integer...

fix rdar://9289603 - clang should fold trivial ?: for enums as well as integer constants into select at -O0
by making the isCheapEnoughToEvaluateUnconditionally predicate handle anything that folds to a constant.  In particular, we now fold enums.

llvm-svn: 129649