Basic/Diagnostics: Rewrite DiagnosticIDs::getDiagnosticLevel completely to be...

Basic/Diagnostics: Rewrite DiagnosticIDs::getDiagnosticLevel completely to be straighter line code, use the new DiagnosticMappingInfo flags, and eliminate the odd MAP_WARNING_NO_WERROR and friend mappings.
 - This fixes a host of obscure bugs with regards to how warning mapping options composed with one another, and I believe makes the code substantially easier to read and reason about.

llvm-svn: 140770