[libc] Add fully-qualified target names.

Only targets setup by the special LLVM libc rules now have fully
qualified names. The naming style is similar to fully qualified names in
Python.

Reviewers: abrachet, PaulkaToast, phosek

Differential Revision: https://reviews.llvm.org/D77340

GitOrigin-RevId: e4767a6f1435164e6eb65c71a0a847812390a55e
33 files changed