[llvm] [libc][bazel] Add a helper library with all deps of generated headers. (PR #136582)

[Alexey Samsonov via llvm-commits]

vonosmas wrote:

Yes, it builds and runs successfully, because this test only verifies the implementation of llvm-libc-macros/stdbit-macros.h, not the validity of a generated header.

https://github.com/llvm/llvm-project/pull/136582