[incrParse] Add warning not to add anything to the identifier-at-eof test

Author: ahoppen

test/incrParse/extend-identifier-at-eof.swift

@@ -1,4 +1,6 @@
 // RUN: %incr-transfer-tree --expected-incremental-syntax-tree %S/Outputs/extend-identifier-at-eof.json %s
 
 func foo() {}
+// ATTENTION: This file is testing the EOF token. 
+// DO NOT PUT ANYTHING AFTER THE CHANGE, NOT EVEN A NEWLINE
 _ = x<<<|||x>>>