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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

The error mentioned above is still present. I haven't looked too much into fixing it. It looks like there have been a couple other regression in regards to warnings in the mean time as well looking at some recent runs.

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