Corrrect warn_unused_result attribute

The original idea was that if the attribute on an operator, 
that the return-value unused-ness wouldn't matter. However, 
all of the operators except postfix inc/dec return 
references! References don't result in this warning 
anyway, so those are already excluded.

Differential Revision: https://reviews.llvm.org/D32207

llvm-svn: 300764