Refactor constant expression handling and make a couple of tweaks to make it a

Refactor constant expression handling and make a couple of tweaks to make it a
bit more robust against future changes. This includes a slight diagnostic
improvement: if we know we're only trying to form a constant expression, take
the first diagnostic which shows the expression is not a constant expression,
rather than preferring the first one which makes the expression unfoldable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194098 91177308-0d34-0410-b5e6-96231b3b80d8