[llvm] [CI] Disable flang from pre-commit tests when Flang files are not touched (PR #93485)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

> Now, of course, flang depends on both, but it's also no officially in production, so random crashes are still expected until it's as stable and complete as clang.

This is a bit of a weird take to me: what does "officially in production" mean? Did MLIR ever got "officially in production" in the context of the LLVM project? The only thing I'm aware of is the notion of "experimental", which applies to backends (Flang was kind of in-between at the beginning because it was C++17 before LLVM was, but it's all aligned now).
There are degrees of maturity of course, but I haven't seen issues with the Flang-on-Linux CI so far (some frontend files are annoyingly heavy to compile!), or only very occasionally (flaky tests can happen, but not more than in LLVM itself or any other project)

https://github.com/llvm/llvm-project/pull/93485