Teach -Wunsequenced that the side-effects of a function evaluation are sequenced

Teach -Wunsequenced that the side-effects of a function evaluation are sequenced
before the value computation of the result. In C, this is implied by there being
a sequence point after their evaluation, and in C++, it's implied by the
side-effects being sequenced before the expressions and statements in the
function body.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185282 91177308-0d34-0410-b5e6-96231b3b80d8