[clang] [Modules] no_undeclared_includes modules (Apple Darwin) don't work the clang modules (PR #68241)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

I could also revert most of the test changes and augment no-undeclared-includes-builtins.cpp, but I liked having a dedicated test that's not necessarily specific to Apple or glibc to test the intended system module setups with `-fbuiltin-headers-in-system-modules` set and unset. Let me know if you feel like the tests here are too redundant.

https://github.com/llvm/llvm-project/pull/68241