[clang][NFC] Remove unreachable code

[urnathan]

Summary

NamespaceDecls are NamedDecls, so NSD can never be non-null in the else branch. Add a comment about this whole ModuleInternal linkage concept going away when p1815 is implemented.

[bruno]

LGTM

clang/lib/AST/Decl.cpp

Module *Decl::getOwningModuleForLinkage(bool IgnoreLinkage) const {
   case Module::ModuleInterfaceUnit:
     return M;
 
   case Module::GlobalModuleFragment: {
     // External linkage declarations in the global module have no owning module
     // for linkage purposes. But internal linkage declarations in the global
     // module fragment of a particular module are owned by that module for
     // linkage purposes.
+    // FIXME: p1815 removes the need for this distinction -- there are no
+    // internal linkage declarations that need to be referred to from outside
+    // this TU.
     if (IgnoreLinkage)
       return nullptr;
     bool InternalLinkage;
     if (auto *ND = dyn_cast<NamedDecl>(this))
       InternalLinkage = !ND->hasExternalFormalLinkage();
-    else {
-      auto *NSD = dyn_cast<NamespaceDecl>(this);
-      InternalLinkage = (NSD && NSD->isAnonymousNamespace()) ||
-                        isInAnonymousNamespace();
-    }
+    else
+      InternalLinkage = isInAnonymousNamespace();
     return InternalLinkage ? M->Parent : nullptr;
   }
 
   case Module::PrivateModuleFragment:
     // The private module fragment is part of its containing module for linkage
     // purposes.
     return M->Parent;
   }