-Wunused-parameter: Don't fire on defaulted or deleted functions

Patch by Dinesh Dwivedi!

Differential Revision: http://reviews.llvm.org/D3376

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@207672 91177308-0d34-0410-b5e6-96231b3b80d8