[NFC] [C++20] [Modules] Test if the functions in importee are generated

In O0, all the functions (except the always-inline-functions) in the importee
shouldn't be imported for compilation speeds.

But with optimizations, all the potentially called function in the
importee should be imported to not prevent any inter-procedural
optimizations (primarily inline), which is pretty important for runtime
performances.

This patch adds the tests for the feature.