Revert "Speculative fix for va_list/nullability test on Hexagon and PPC."

This reverts commit r286533. At this point an array really is still an
array, but the problem is with /non-/array va_lists anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286541 91177308-0d34-0410-b5e6-96231b3b80d8