[patch] Check for isDiagnosticEnabled even if we have a dig handler

[Rafael Espíndola]

> We had the same problem in libLTO, and I added `RespectDiagnosticFilters` in
> r218784 to avoid breaking anyone using the old behaviour.  Maybe that wasn't
> important -- I may have been too conservative? -- but we should be consistent.
> (Maybe I did this because of `clang`... does it have its own filters?)

Oh, I missed that. I did the minimal fix in r231132.

RespectDiagnosticFilters does look like more flexibility than
necessary, but that is independent. I will try writing a patch to see
what breaks.

Cheers,
Rafael