Don't shadow locals, parameters or fields

Don't shadow locals, parameters or fields

Summary:
This accounts for the places that were triggering warnings 4456, 4457, and 4458, which are all related to shadowing names, be they locals, parameters, or even types.
This doesn't deal with 4459, which is specifically for shadowing global variables, because folly/gen defines globals by the name of `count`, `min`, `max` and a few other similar names.

Reviewed By: meyering

Differential Revision: D4296781

fbshipit-source-id: a2e625095e2c65a53a9226b000aaf0ca95a3a393