Properly account for redeclarations when explicitly instantiating class templates.

What happens here is that we actually turn the first declaration into a
definition, regardless of whether it was actually originally a definition,
and furthermore we do this all after we've instantiated all the declarations.
This exposes a bug in my DefinitionData patch where it was only setting the
DefinitionData for previous declarations, not future declarations.
Fortunately, there's an iterator for that.

llvm-svn: 99657