Removing some unneeded code, and a diagnostic that was obsoleted. The type has already been determined to be a ValueDecl by virtue of the attribute subjects.

Added some test case coverage as well.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198046 91177308-0d34-0410-b5e6-96231b3b80d8