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

include/swift/Parse/Parser.h

@@ -1097,6 +1097,7 @@ public:
                                               bool isExprBasic);
   ParserResult<Expr> parseExprPostfix(Diag<> ID, bool isExprBasic);
   ParserResult<Expr> parseExprUnary(Diag<> ID, bool isExprBasic);
+  ParserResult<Expr> parseExprSelector();
   ParserResult<Expr> parseExprSuper();
   ParserResult<Expr> parseExprConfiguration();
   Expr *parseExprStringLiteral();

include/swift/Parse/Token.h

@@ -44,6 +44,7 @@ enum class tok {
   pound_endif,
   pound_line,
   pound_available,
+  pound_selector,
   comment,
   
 #define KEYWORD(X) kw_ ## X,