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

[Chandler Carruth via llvm-commits]

chandlerc wrote:

> Could you clarify what/how you're trying to build? E.g. we have an llvm-libc BUILD target (cc_library) here: [`main`/utils/bazel/llvm-project-overlay/libc/BUILD.bazel#L1817-L1831](https://github.com/llvm/llvm-project/blob/main/utils/bazel/llvm-project-overlay/libc/BUILD.bazel#L1817-L1831)
> 
> that is used in downstream Bazel build of libcxx (it's simply listed as a dependency of proper rules for libcxx).

I'm trying to assemble a package with Bazel that has all the sources needed to build libc++, not trying to build libc++ itself.

The goal is to allow Carbon to support building runtimes on-demand in all cases, so we're installing all the source files and building in build steps that we'll run automatically as needed.

>From some discussions with other llvm-libc folks, it seemed like using the sources directly was an intended and supported use case (and that matches having the file groups for the dependencies), so I was sending this PR to somewhat complete the picture. But can also keep this out of tree if needed.

I don't quite have the last patches for libc++ polished, but happy to point folks to them once they are.

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