SE-0022: Implement parsing, AST, and semantic analysis for #selector.

lib/IDE/SyntaxModel.cpp

@@ -98,6 +98,7 @@ SyntaxModelContext::SyntaxModelContext(SourceFile &SrcFile)
 #define KEYWORD(X) case tok::kw_##X: Kind = SyntaxNodeKind::Keyword; break;
 #include "swift/Parse/Tokens.def"
 #undef KEYWORD
+      case tok::pound_selector: Kind = SyntaxNodeKind::Keyword; break;
       case tok::pound_line:
       case tok::pound_available: Kind =
           SyntaxNodeKind::BuildConfigKeyword; break;