Implement support for [[nodiscard]] in C++1z that is based off existing support for...

Implement support for [[nodiscard]] in C++1z that is based off existing support for warn_unused_result, and treat it as an extension pre-C++1z. This also means extending the existing warn_unused_result attribute so that it can be placed on an enum as well as a class.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262872 91177308-0d34-0410-b5e6-96231b3b80d8