Enhance ClangDiagnosticsEmitter to reject diagnostics that are errors that are also

included in warning groups.  Warning groups can only contain warnings, because only
warnings can be mapped to errors or ignored.

This caught a few diagnostics that were incorrectly in diagnostic groups, and
could have resulted in a compiler crash when those diagnostic groups were mapped.

Fixes <rdar://problem/12044436>

llvm-svn: 161389