Force saving the module name for unsymbolized frames

In some cases pprof isn't saving the module name in graph nodes, so
reports end up with nodes of the form '<unknown>'.

The module name is often stripped to allow symbolized frames from
different versions of a binary to be merge.

Force saving the objfile name if the name is unknown so we can print
some information for unsymbolized frames.

It seems these do not need to be exported.

This is to keep the new TrimTree functionality from breaking any code
currently using the public interface. We do this by separating NodeSet
from nodePtrSet and creating different functions for each.