Make isVisibleFromModule accept TagDecl definitions that were demoted to declarations.
Make isVisibleFromModule accept TagDecl definitions that were demoted to declarations.

Clang AST requires to have only a single definition in a redeclaration
chain, that's why it demotes duplicate definitions from other modules to
declarations. But on the Swift side it is important to know a module
contains a full definition and not just a forward declaration, even if
it was demoted to a declaration.

JGiola/swift,JGiola/swift,gregomni/swift,atrick/swift,JGiola/swift,ahoppen/swift,atrick/swift,apple/swift,ahoppen/swift,benlangmuir/swift,apple/swift,roambotics/swift,ahoppen/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,glessard/swift,atrick/swift,roambotics/swift,apple/swift,JGiola/swift,apple/swift,rudkx/swift,benlangmuir/swift,JGiola/swift,glessard/swift,roambotics/swift,ahoppen/swift,glessard/swift,rudkx/swift,gregomni/swift,ahoppen/swift,benlangmuir/swift,glessard/swift,apple/swift,atrick/swift,apple/swift,roambotics/swift,atrick/swift,glessard/swift,rudkx/swift,rudkx/swift,ahoppen/swift,atrick/swift,benlangmuir/swift,glessard/swift,rudkx/swift,benlangmuir/swift,gregomni/swift,gregomni/swift,gregomni/swift,rudkx/swift,roambotics/swift,gregomni/swift