[NFC] [C++20] [Modules] Add merge-records test for C++20 Modules

Merging declarations and checking ODR violations are a big part in the
implementation of modules. Currently, the implementation of C++20
Modules share a big part with the Clang modules and the ObjC modules for
the clang's serializer/deserailizer.

This is good. We saved a lot of time by this. And a lot of C++20
Modules's codes can run due to reuse it. However, the bad side is that
we lack a lot of tests about merging declarations and ODR checks for
C++20 Modules. It is not stable on the long run. So the patch tries to
add a test for it. This should be a long-term goal for C++20 modules.
(To add the test we shoudl add).