[AST] Correctly set LAST_EXPR

Author: hamishknight

include/swift/AST/ExprNodes.def

@@ -213,7 +213,8 @@ UNCHECKED_EXPR(OneWay, Expr)
 EXPR(Tap, Expr)
 UNCHECKED_EXPR(TypeJoin, Expr)
 EXPR(MacroExpansion, Expr)
-LAST_EXPR(TypeJoin)
+// Don't forget to update the LAST_EXPR below when adding a new Expr here.
+LAST_EXPR(MacroExpansion)
 
 #undef EXPR_RANGE
 #undef LITERAL_EXPR