This patch adds a UseCounter for each of the following:
- Allowing duplicate sloppy-mode block-scoped function declarations
  in the exact same scope
- for-in loops with an initializer

The patch also refactors some of the declaration code to clean it up and
enable the first counter, and adds additional unit tests to nail down
the semantics of edge cases of sloppy-mode block-scoped function declarations.

BUG=v8:4693,chromium:579395
LOG=N
R=adamk

Review URL: https://codereview.chromium.org/1633743003

Cr-Commit-Position: refs/heads/master@{#33650}