Enable C++11 attribute syntax for warn_unused_result and allow it to be

Enable C++11 attribute syntax for warn_unused_result and allow it to be
applied to CXXRecordDecls, where functions with that return type will
inherit the warn_unused_result attribute.

Also includes a tiny fix (with no discernable behavior change for
existing code) to re-sync AttributeDeclKind enum and
err_attribute_wrong_decl_type with warn_attribute_wrong_decl_type since
the enum is used with both diagnostic messages to chose the correct
description.

llvm-svn: 167783