[llvm] [bazel] Add filegroup for builtin_headers (PR #67757)

[David Zbarsky via llvm-commits]

dzbarsky wrote:

Oh I guess I misread and it won't change with your PR, I would probably need to update to use the const though. Either way, I think the point is the filegroup is simpler and faster.

https://github.com/llvm/llvm-project/pull/67757