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

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

https://buildkite.com/llvm-project/github-pull-requests/builds/65498#018f95c2-a52b-4d95-97ac-ea7e0c7be9ae confirms that Flang is no longer compiler, and Flang tests are no longer executed.

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