Moved the list of found namespaces into the search

context object.  Having it populated and registered
within a single FindExternalVisibleDecls call worked
fine when there was only one call (i.e., when we were
just looking in the global namespace).  

However, now FindExternalVisibleDecls is called for
nested namespaces as well, which means that it is
called not once but many times (once per module in
which the parent namespace appears).  This means that
the namespace mapping is built up across many calls
to the inferior FindExternalVisibleDecls, so I moved
it into a data structure (the search context) that is
shared by all calls.

I also added some logging to make it easier to see
what is happening during a namespace search, and 
cleaned up some existing logging.

llvm-svn: 141888