[SourceKit] Add an operator syntax kind

This will allow us to do semantic highlighting for operators in SourceKit-LSP.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -452,6 +452,8 @@ UIdent SwiftLangSupport::getUIDForSyntaxNodeKind(SyntaxNodeKind SC) {
   case SyntaxNodeKind::Identifier:
   case SyntaxNodeKind::DollarIdent:
     return KindIdentifier;
+  case SyntaxNodeKind::Operator:
+    return KindOperator;
   case SyntaxNodeKind::Integer:
   case SyntaxNodeKind::Floating:
     return KindNumber;