fixUnusedIdentifier may remove a used side-effect

**TypeScript Version:** 2.9.0-dev.20180506 **Code** ```ts { const x = launchMissiles(); } ``` **Expected behavior:** `const x = ` removed, and missiles still launched. **Actual behavior:** Missiles unlaunched. Unfortunately we have no way of knowing whether a function call does have side effects (#17181), so doing this in general might be annoying as users have to manually delete function calls with unused results.