Add ModuleDecl::ReverseFullNameIterator

Package up the logic that generates a full Clang module name, so that
(a) we don't have to deal with clang::Module in quite as many places
in the /Swift/ compiler, and (b) we can avoid the cost of a temporary
string in a few places.

The main places where this is /not/ adopted is where we don't just
want to know the parent module name, but actually the module itself.
This is mostly indexing-related queries, which use the very similar
ModuleEntity class also defined in Module.h. I didn't quite see an
obvious way to unify these, but that might be where we want to go.

No functionality change.

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -416,21 +416,9 @@ static bool initDocEntityInfo(const Decl *D,
     case DeclContextKind::FileUnit: {
       if (auto *CD = D->getClangDecl()) {
         if (auto *M = CD->getImportedOwningModule()) {
-          const clang::Module *Root = M->getTopLevelModule();
-
-          // If Root differs from the owning module, then the owning module is
-          // a sub-module.
-          if (M != Root) {
+          if (M->isSubModule()) {
             llvm::raw_svector_ostream OS(Info.SubModuleName);
-            llvm::SmallVector<StringRef, 4> Names;
-
-            // Climb up and collect sub-module names.
-            for (auto Current = M; Current != Root; Current = Current->Parent) {
-              Names.insert(Names.begin(), Current->Name);
-            }
-            OS << Root->Name;
-            std::for_each(Names.begin(), Names.end(),
-                          [&](StringRef N) { OS << "." << N; });
+            ModuleDecl::ReverseFullNameIterator(M).printForward(OS);
           }
         }
       }