[SourceKit] Add a request to get the semantic tokens of a file

We need this request for semantic highlighting in LSP. Previously, we were getting the semantic tokens using a 0,0 edit after a document update notification but that will no longer be possible if we open the documents in syntactic only mode.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -397,6 +397,10 @@ UIdent SwiftLangSupport::getUIDForRefactoringKind(ide::RefactoringKind Kind){
   }
 }
 
+UIdent SwiftSemanticToken::getUIdentForKind() {
+  return SwiftLangSupport::getUIDForCodeCompletionDeclKind(Kind, IsRef);
+}
+
 UIdent SwiftLangSupport::getUIDForCodeCompletionDeclKind(
     ide::CodeCompletionDeclKind Kind, bool IsRef) {
   if (IsRef) {