[libc] Add cmake target for linting libc.

Summary:
This patch implements running linting on llvm-libc using build rule targets.

1) adds a new target per entrypoint for linting with the naming convention `<qualified_target_name>.__lint__` e.g `libc.src.string.strlen.__lint__`.
2) makes the build target for each entrypoint depend on the linting targets so that they run along with compilation of each entrypoint.
3) adds a lint all target named `lint-libc`.  `check-libc` now depends on this new target.
4) linting creates a lot of additional targets from clang and clang-tidy that need to be built so an opt out flag can be passed to cmake: `LLVM_LIBC_ENABLE_LINTING`.

Reviewers: sivachandra, abrachet

Reviewed By: sivachandra

Subscribers: abrachet, mgorny, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D77861