Add -Wunsequenced (with compatibility alias -Wsequence-point) to warn on
expressions which have undefined behavior due to multiple unsequenced
modifications or an unsequenced modification and use of a variable.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172690 91177308-0d34-0410-b5e6-96231b3b80d8