Adding pre/post conditions for some Replacement handling functions

The vector version of shiftedCodePosition() requires Replacements to be
in sorted order.

Turned existing comment about sorting for deduplicate() into an exact
post condition.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191536 91177308-0d34-0410-b5e6-96231b3b80d8