[clang] [llvm] Disable compiling Flang and running its tests on Clang changes (PR #92740)

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

Windows CI has finished with an unrelated failure. https://buildkite.com/llvm-project/github-pull-requests/builds/65498 confirms that this patch is working as intended, i.e. Flang is not tested on Clang changes on both Linux and Windows.

https://github.com/llvm/llvm-project/pull/92740