[Modules] Don't judge if we're compiling a module unit by LangOpts::CurrentModule.empty()

Closing https://github.com/llvm/llvm-project/issues/57778.

Previously it judge if we're compiling a module unit by
LangOpts::CurrentModule.empty(). But it is not true since we can specify
the module name by `-fmodule-name` option for arbitrary module unit.
Then this patch adjuest the judgement properly.