Sema: Implement DR477

Summary: Friend declarations shouldn't mention explicit or virtual.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3562

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207682 91177308-0d34-0410-b5e6-96231b3b80d8