While this won't help fix things like the bug that r260219 addressed, it
seems like good tidy up to have anyway.

(it might be nice if "makeArrayRef" always produced a MutableArrayRef &
let it decay to an ArrayRef when needed - then I'd use that for the
MutableArrayRefs in this patch)

If we had std::dynarray I'd use that instead of unique_ptr+size_t,
ideally (but then it'd have to be threaded down through the Preprocessor
all the way - no idea how painful that would be)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260246 91177308-0d34-0410-b5e6-96231b3b80d8