PR16467: Teach -Wunsequenced that in C11 (unlike C++11), an assignment's
side-effect is not sequenced before its value computation. Also fix a
mishandling of ?: expressions where the condition is constant that was
exposed by the tests for this.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185035 91177308-0d34-0410-b5e6-96231b3b80d8