[NFC] Cleanup Dex

* Use consistent assertion messages in iterators implementations
* Silence a bunch of clang-tidy warnings: use `emplace_back` instead of
  `push_back` where possible, make sure arguments have the same name in
  header and implementation file, use for loop over ranges where possible

Reviewed by: ioeric

Differential Revision: https://reviews.llvm.org/D51528

llvm-svn: 341190