Adding the AddOverride transform for cpp11-migrate

This transform adds the override specifier to methods that overrides virtual
methods from a base class that don't already have this specifier.

Author: Philip Dunstan <phil@phildunstan.com>


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@179127 91177308-0d34-0410-b5e6-96231b3b80d8