Move the warning about unused relational comparison from -Wunused-value to
-Wunused-comparison.  Also, newly warn on unused result from overloaded
relational comparisons, now also in -Wunused-comparison.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203535 91177308-0d34-0410-b5e6-96231b3b80d8