Once we have fully deserialized a redeclaration chain for something

with a definition pointer (e.g., C++ and Objective-C classes), zip
through the redeclaration chain to make sure that all of the
declarations point to the definition data. 

As part of this, realized again why the first redeclaration of an
entity in a file is important, and brought back that idea.

llvm-svn: 146886