cpp11-migrate: Adding -yaml-only option

For use with -headers, -yaml-only will cause cpp11-migrate to not write header
changes to disk and instead write them as header change description files. This
option facilitiates upcoming functionality to properly support changing headers
as part of migration.

Differential Revision: http://llvm-reviews.chandlerc.com/D1385



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188371 91177308-0d34-0410-b5e6-96231b3b80d8