[libc++] Build and test with -Wundef warning. NFC.

This will avoid typos like `_LIBCPP_STD_VERS` (<future>) or using `#if TEST_STD_VER > 17` without including "test_macros.h".

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D99515

GitOrigin-RevId: 5c703f0fd81960386cc02b9ca618e866c9d5e29a
12 files changed