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

[Alexey Samsonov via llvm-commits]

vonosmas wrote:

Could you clarify what/how you're trying to build? E.g. we have an llvm-libc BUILD target (cc_library) here: 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).

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