[regexp] Add file-level typedefs to builtins-regexp.cc

This removes lots of boilerplate typedefs, with the caveat of renaming
CSA::Label and Variable to CLabel and CVariable within builtins-regexp.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2524363002
Cr-Commit-Position: refs/heads/master@{#41281}