[libc][cmake] Warn when the main header is missing while its entrypoint is included

It's easy to forget to enable a header when enabling its (first) entry point. This would help us detect that.

See https://github.com/llvm/llvm-project/pull/210008#pullrequestreview-4714561358 for context.

Start by reading the linked pull request review for the intended behavior, then trace the libc CMake logic that enables headers and their first entrypoints. Confirm the relevant configuration warns when an entrypoint is enabled without its main header, and verify the warning through the applicable CMake checks.

Written by the indexing model from the issue text.