DiagnosticsEngine: update severity setters to new terminology

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210764 91177308-0d34-0410-b5e6-96231b3b80d8
Fix msvc unittest build.

Looks like msvc has an asymmetrical operator ==.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210768 91177308-0d34-0410-b5e6-96231b3b80d8