[libc++][modules] Improves export validation.

Adds tests to guard against the issue solved in D156592.

Depends on D156592

Reviewed By: ChuanqiXu, #libc, philnik

Differential Revision: https://reviews.llvm.org/D156594

GitOrigin-RevId: bae14bcd1d9eba2cfae0d755c9d6b61e6ed3a6ca