Close https://github.com/llvm/llvm-project/issues/71347
Previously I misread the concept of module purview. I thought if a
declaration attached to a unnamed module, it can't be part of the module
purview. But after the issue report, I recognized that module purview is
more of a concept about locations instead of semantics.
Concretely, the things in the language linkage after module declarations
can be exported.
This patch refactors `Module::isModulePurview()` and introduces some
possible code cleanups.
0f7aaeb324