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

[Chandler Carruth via llvm-commits]

chandlerc wrote:

> Do you also assemble a package with the sources needed to build llvm-libc? If yes, do you use smth. like `libc_release_library` with a list of functions that the Carbon needs? I'm asking because we do have some concerns about the scalability of the current approach in the downstream builds -- where Bazel has to analyze hundreds / thousands of targets from runtimes-on-demand libc for every single build.

I've not looked at shipping libc itself yet, just libc++ and libunwind. Long term, definitely interested, but libc++ was specifically blocking work and so was prioritizing that.

The current dependency isn't particularly noticeable for me yet.

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