Introduce Replacement deduplication and conflict detection function

Summary:
This patch adds tooling::deduplicate() which removes duplicates from and
looks for conflicts in a vector of Replacements.

Differential Revision: http://llvm-reviews.chandlerc.com/D1314



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187979 91177308-0d34-0410-b5e6-96231b3b80d8