[C++20] [Modules] Judge current module correctly

Now the implementation would accept following code:
```
//--- impl.cppm
module M:impl;
class A {};

//--- M.cppm
export module M;
import :impl;

//--- Use.cpp
import M;
void test() {
    A a; // Expected error. A is not visible here.
}
```

which is clearly wrong.  The root cause is the implementation of
`isInCurrentModule` would return true if the module is a partition! So
in the above example, although Use.cpp is not a module unit,
`isInCurrentModule ` would still return true when the compiler tries to
see if the owning module of `A` is the current module. I believe this is
an oversight. This patch tries to fix this problem.

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D123837