[llvm] Add a filegroup containing _all_ sources to the libc build rules (PR #169155)

[Michael Jones via llvm-commits]

michaelrj-google wrote:

If you're just trying to build libc++ you shouldn't need to add a new target, you should be able to use the `libcxx_shared_headers` target: https://github.com/llvm/llvm-project/blob/main/utils/bazel/llvm-project-overlay/libc/BUILD.bazel#L1810

https://github.com/llvm/llvm-project/pull/169155