clang-rename: split existing options into two new subcommands

- rename-at is meant to be integrated with editors and works mainly off
  of a location in a file, and this is the default
- rename-all is optimized for one or more oldname->newname renames, and
  works with clang-apply-replacements

Reviewers: bkramer, klimek

Subscribers: omtcyfz

Differential Revision: https://reviews.llvm.org/D21814

llvm-svn: 277438