[IDE][AST] Handle frameworks with traditional overlays where the underlying module declares cross imports in sourcekit.

We weren't handling this case, so their generated interfaces / doc info
wouldn't include symbols from the cross-import overlays, and we wouldn't
map the underscored cross-import overlay name back to the declaring
framework's name in cusor-info, completion results or when indexing.

Resolves rdar://problem/62138551

lib/Serialization/SerializedModuleLoader.cpp

@@ -1218,6 +1218,10 @@ StringRef SerializedASTFile::getTargetTriple() const {
   return File.getTargetTriple();
 }
 
+ModuleDecl *SerializedASTFile::getUnderlyingModuleIfOverlay() const {
+  return File.getUnderlyingModule();
+}
+
 const clang::Module *SerializedASTFile::getUnderlyingClangModule() const {
   if (auto *UnderlyingModule = File.getUnderlyingModule())
     return UnderlyingModule->findUnderlyingClangModule();