[llvm] [Bazel] Make LLVM and Clang config headers configurable (PR #126729)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Making it configurable is definitely an improvement:) I am not familiar with Bazel.

I'd recommend getting explicit approval from @rupprecht.

https://github.com/llvm/llvm-project/pull/126729