Reland https://github.com/llvm/llvm-project/pull/75912
The differences of this PR between
https://github.com/llvm/llvm-project/pull/75912 are:
- Fixed a regression in `Decl::isInAnotherModuleUnit()` in DeclBase.cpp
pointed by @mizvekov and add the corresponding test.
- Fixed the regression in windows
https://github.com/llvm/llvm-project/issues/97447. The changes are in
`CodeGenModule::getVTableLinkage` from
`clang/lib/CodeGen/CGVTables.cpp`. According to the feedbacks from MSVC
devs, the linkage of vtables won't affected by modules. So I simply
skipped the case for MSVC.
Given this is more or less fundamental to the use of modules. I hope we
can backport this to 19.x.
