Make Attr::Clone() also clone the Inherited, IsPackExpansion and Implicit flags

I was bitten by this when working with the dll attributes: when a dll
attribute was cloned from a class template declaration to its
specialization, the Inherited flag didn't get cloned.

Differential Revision: http://reviews.llvm.org/D3972

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209950 91177308-0d34-0410-b5e6-96231b3b80d8