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

[Kiran Chandramohan via llvm-commits]

kiranchandramohan wrote:

> 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.

This warning is also fixed by the following.
https://github.com/llvm/llvm-project/pull/81899

Thanks for your patience.

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