Solve Entry merger crash (LT-14819)

* Write test for ReplaceReference
* Write test for FwFixFwdataDll
* Fix code in ReplaceReference so that replacing a single
  Target in a LexReference doesn't delete the LexReference
  (and crash)
* Add code to the Fixer so that LexReferences with
  single targets get deleted

Change-Id: If77ffbd496b86b3e10c30995f470667d8767c0b8