Factor the merging of declarations in the AST reader out to a separate

member function template, since the behavior is identical for
ObjCInterfaceDecl and ObjCProtocolDecl. It's expected that all
redeclarable entities will have the same behavior.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147450 91177308-0d34-0410-b5e6-96231b3b80d8