Adding a -Wunused-value warning for expressions with side effects used in an unevaluated expression context, such as sizeof(), or decltype(). Also adds a similar warning when the expression passed to typeid() *is* evaluated, since it is equally likely that the user would expect the expression operand to be unevaluated in that case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224465 91177308-0d34-0410-b5e6-96231b3b80d8