[C++20] [Modules] Support to export declarations in language linkage

Close https://github.com/llvm/llvm-project/issues/60405

See the discussion in the above link for the background.

What the patch does:
- Rename `Module::ModuleKind::GlobalModuleFragment` to
  `Module::ModuleKind::ExplicitGlobalModuleFragment`.
- Add another module kind `ImplicitGlobalModuleFragment` to
  `ModuleKind`.
- Create an implicit global module fragment for the language linkage
  declarations inside a module purview.
    - If the language linkage lives inside the scope of an export decl,
      the created modules is marked as exported to outer modules.
- In fact, Sema will only create at most 2 implicit global module
  fragments to avoid creating a lot of unnecessary modules in the edging
case.

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D144367