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

[Kiran Chandramohan via llvm-commits]

kiranchandramohan wrote:

> We'll probably need to wait a little bit before merging this, but there's only one final warning that I can see before this is ready to merge:
> 
> ```
> /tmp/llvm-project/build/tools/flang/docs/Source/FlangCommandLineReference.rst:176: WARNING: unknown document: 'DiagnosticsReference'
> ```
> 
> Once that gets fixed, we should be able to get this in.

This warning (and all others) are now fixed. 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]
```

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