[llvm] [CI] Disable Flang from pre-commit tests when Flang files are not touched on Windows Only (PR #93729)

[Renato Golin via llvm-commits]

rengolin wrote:

> What would be a reasonable state for the Windows Flang builds to be re-enabled for LLVM and MLIR? Demonstration of One week or month without OOM in the flang patches (where the windows flang build and tests run)?

Our [new target policy](https://llvm.org/docs/DeveloperPolicy.html#introducing-new-components-into-llvm) states `3 months` of stability, but that's an old policy for simpler times.

My take on this is that Flang is already an integral part of LLVM, but currently has unstable testing. I propose we follow the original buildbot testing convention to _"move noisy bots to silent until they are deemed stable again"_ repeatedly, every time it gets noisy again. How long? It depends on what went wrong.

Let's say this time you fix the problem, wait a few weeks, then add to pre-commit CI again. If in a few months we start having problems again (reported, not fixed), I think it's perfectly reasonable for anyone (non-Flang developers) to re-land this same patch and open an issue to Flang owners to fix, restabilize, re-enable.

This should be a natural state of our pre-commit CI, not just for Flang or Windows, but for everything that causes grief to unrelated developers.

https://github.com/llvm/llvm-project/pull/93729