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

[David Zbarsky via llvm-commits]

dzbarsky 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

I'm building the entire tar package within Bazel and I'm not packaging clang-tidy (you don't need it for a minimal toolchain) so I don't think it helps.

> > 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

I think something about my compilation setup is non-hermetic, because when executing from a github actions runner (with RBE/remote caching) I am seeing tons of remote misses. When running from a minimal docker image on my laptop (also against RBE) I am seeing everything cached, so I suspect we are somehow leaking the repo path, which is probably randomized on github. Anyway, this is a total aside, but the caching doesn't work as well as I'd like :)

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