libclang_rt: Descend into libclang_rt to clean

[markj]

Summary

I discovered that during cleanworld, we don't descend into libclang_rt.

[dim]

I think this makes sense, thanks. Proves how often I run cleanworld, which is almost never :)

[markj]

In D50766#1158776, @dim wrote:

I think this makes sense, thanks. Proves how often I run cleanworld, which is almost never :)

This also affects buildworld WITH_CLEAN=. I noticed because I set some build flags to avoid embedding source paths into object files when WITH_REPRODUCIBLE_BUILD is defined, but they had no effect on certain files in libclang_rt even after a clean buildworld.