[clang-tidy] Format code around applied fixes

Summary:
Add -format option (disabled by default for now) to trigger formatting
of replacements.

Reviewers: ioeric

Reviewed By: ioeric

Subscribers: kimgr, malcolm.parsons, JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D30564

llvm-svn: 296864