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

[via llvm-commits]

lntue 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.

Then can we add some comments on the file group mentioning that this is used in Carbon's build, so that it will not be cleaned up in the future, and when these need to be changed, we will know which downstream project to ask.

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