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

[David Zbarsky via llvm-commits]

dzbarsky wrote:

I could. It would require the following change on my side:
```
 pkg_files(
     name = "builtin_headers_pkg_files",
     srcs = [
-        "@llvm-project//clang:builtin_headers_files",
+        "@llvm-project//clang:builtin_headers_gen",
     ],
-    strip_prefix = "lib/Headers",
+    strip_prefix = strip_prefix.from_pkg() + "staging/include/",
     prefix = "lib/clang/%s/include" % LLVM_VERSION_MAJOR,
```

This seems more brittle, it will break with your PR. 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?

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