More constant evaluation cleanup, and fix an issue where we'd override an
earlier 'non-constant' diagnostic with a later one if the earlier one was from
a side-effect we thought we could evaluate past.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194117 91177308-0d34-0410-b5e6-96231b3b80d8