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

[Chandler Carruth via llvm-commits]

chandlerc wrote:

FYI, a PR patching this in and using it is here: https://github.com/carbon-language/carbon-lang/pull/6424

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