[flang][driver] Avoid mentions of Clang in Flang's command line reference. (#88932)

The help text was not updated in #87360.

Clang is also mentioned for the diagnostic warnings reference, which
mostly applies to C/C++/Obj-C, not Fortran. #81726 already tried to fix
this, and I don't know a better solution.

GitOrigin-RevId: fdc8c5440041ac53726d0b3587762ceeb8cbbb4f
1 file changed