feat: Support -Xpreprocessor -fopenmp in direct mode

feat: Support -Xpreprocessor -fopenmp in direct mode

All uses of -Xpreprocessor disable the direct mode as a safety measure
since the command line could include things like

  -Xpreprocessor -MF -Xpreprocessor file.d

which ccache needs to understand as

  -MF file.d

Ideally, ccache should handle this in a generic way. Meanwhile, let's
allow it in the a special case of "-Xpreprocessor -fopenmp" since that's
required on macOS to enable OpenMP.

Closes #1434.