libclang: remove 'CXDiagnostic_Remark'

The change was landed without review or test cases.

It trivially broke almost any stable application checking for Severity >=
CXDiagnostic_Error or indeed any other kind of severity comparison upon
encountering a 'remark'.

Mapped to CXDiagnostic_Warning until a workable solution is proposed to the
list that preserves API stability.

(It's also not clear why the rest of r202475 wasn't simply implemented as a
modifier to the existing 'warning' level.)