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

[David Spickett via cfe-commits]

DavidSpickett wrote:

> The docbrief strings already use %Program/GlobalDocumentation.Program which seems to work and made me think that HelpTextForVariants was actually unnecessary. Would you like me to introduce DocBriefForVariants?

IIRC it was unnessecary for the documentation because it gets built multiple times, for the driver it's built once so it has to include all possible variants the first time.

I have no preference for how it's done, if the `%Program` thing works then fine. We only need a `ForVariants` if it's something people see when interacting with Flang itself I think.

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