[llvm] [Github] Enable warnings as errors on flang sphinx build (PR #72723)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> This warning (and all others) are now fixed. 

There were still two errors popping up with the CI configuration, but it looks like those are due to building the `docs-flang-man` target, which doesn't actually produce anything. Given that's not configured properly, it should probably be removed?

>But I see the following mlir warning that is probably caused because flang needs mlir tools.
> 
> ```
> /home/buildbot/as-worker-4/publish-sphinx-docs/llvm-project/mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp:2956:38: warning: unused variable ‘operand’ [-Wunused-variable]
> ```

I can't reproduce that warning at all with a recent clang version. The CI uses `gcc` (I believe), so it might show up there. It's a compiler warning though, so doesn't impact the viability of this patch.

I'm going to land this soonish as everything should be good to go now. Thanks for your work on fixing this up!

https://github.com/llvm/llvm-project/pull/72723