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

[Michael Kruse via cfe-commits]

Meinersbur wrote:

> Do you mean that this PR fixes this, or that you noticed this problem while working on this?

I noticed this when working on the patch, .i.e that https://flang.llvm.org/docs/FlangCommandLineReference.html still mentioned Clang in several places and since it was common, I didn't want to fix it only for `--gcc-toolchain`. `HelpTextForVariants` was introduced later, reducing the number of "Clang" in FlangCommandLineReference already. This fixes `--gcc-toolchain`, but there is no list of warnings supported by Flang, so I don't have anything beyond what #81726  already did.

> 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.

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`?


> @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.

```
def Diag_Group : OptionGroup<"<W/R group>">, Group<CompileOnly_Group>,
                 DocName<"Diagnostic options">,
                 DocBrief<!strconcat(
                   StringForProgram<"Flags controlling which warnings, errors, and remarks %Program will generate. ">.str,
                   !cond(!eq(GlobalDocumentation.Program, "Clang"):
                       "See the :doc:`full list of warning and remark flags <DiagnosticsReference>`.",
                     true:
                       "See Clang's Diagnostic Reference for a full list of warning and remark flags."
                   )
                 )>;
```


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