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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> > 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.
> 
> Do you mean that this PR fixes this, or that you noticed this problem while working on this?
> 
> If it's the latter it may be as @banach-space mentioned to me, that the `docbrief` is still shared between Flang and Clang. This could be solved in a similar, albeit tedious way to what I did for the help text.

@Meinersbur Could you share an example? If it's a problem specific to diagnostics then we should possibly just start separating Clang and Flang diagnostics. That could mean a bit of duplication, but not too much, so I'm not worried.

https://github.com/llvm/llvm-project/pull/88932