[Clang] Shut up warning.
Clang 6.0.1 does not know -Wbool-operation, and ignoring -Wpragmas is not sufficient to shut it up about this. So ignore -Wunknown-warning-option too.
Clang 6.0.1 does not know -Wbool-operation, and ignoring -Wpragmas is not sufficient to shut it up about this. So ignore -Wunknown-warning-option too.