[libc] Add rule named `add_libc_hermetic_test` which adds a hermetic test.

A convenience wrapper name `add_libc_test` is also added which adds both
a unit test and a hermetic test. The ctype tests have been switched over
to use add_libc_test.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D148756