Add -Wreorder to the list of C++ warnings.

This built clean with clang, but if we see false positives on the bots
then we'll revert and turn it into a compiler specific check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181836 91177308-0d34-0410-b5e6-96231b3b80d8