introduce a new NamedDecl class, switch a couple of things over to using it.

NamedDecl is a Decl that has an IdentifierInfo (for example, ScopedDecl), 
but not ObjcMethodDecl.

Simplify some code in ActOnAddMethodsToObjcDecl, by doing the cast from 
DeclTy to Decl at the start of the method.

llvm-svn: 42710