This is step 4 of https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079
This is just an implementation detail. Reviewed By: sivachandra Differential Revision: https://reviews.llvm.org/D152532
add_libc_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