[C++20] [Modules] Don't generate unused variables in other module units

[C++20] [Modules] Don't generate unused variables in other module units
even if its initializer has side effects

Close https://github.com/llvm/llvm-project/issues/61892

The variables whose initializer has side effects will be emitted even if
it is not used. But it shouldn't be true after we introduced modules.
The variables in other modules shouldn't be emitted if it is not used
even if its initializer has size effects.

Also this patch rename `Decl::isInCurrentModuleUnit` to
`Decl::isInAnotherModuleUnit` to make it closer to the semantics.