Tweak Replacement comparisons

* Introduce operator< to replace Replacement::Less
* Make operator== and operator< on Replacements non-member functions
* Change order of comparisons in operator< to do string comparisons last



git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@188550 91177308-0d34-0410-b5e6-96231b3b80d8