identifierCount may not be correct

**TypeScript Version:** 3.2.0-dev.20181114 In `parser.ts`, there is an `identifierCount` variable that is incremented every time `createIdentifier` is called. Unfortunately, not all `createIdentifier` calls will result in an identifier in the AST, since some may have happened during speculative parsing. It may be necessary to reset `identifierCount` after speculative parsing fails.