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

[via llvm-commits]

jathu wrote:

Do you think you will still need this once we land #67626? The clang-tidy binary will have the header files as runfiles, which you can just package directly


> Also that genrule takes 17 seconds in my setup to copy the files around, which is a waste since I'm just going to package them up anyway. I'd probably stick with my local patch in that case. Any reason not to add this?

No, I don't see any reason not to add this. I assumed the filegroup was redundant since the genrule would cache the subsequent uses anyways

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