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

[Alexey Samsonov via llvm-commits]

vonosmas 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

I believe the Carbon build just constructs a filegroup with libcxx sources (and all their dependencies), hence the need the filegroup with shared libc sources as well. I'm OK with having this in the tree right now, though we clearly don't have a proper build configuration to verify that it'd keep working.

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