C++ PATCH for c++/35255 (address of template-id)

[Jason Merrill]

Per DR 115, if the context of a template-id doesn't give enough type 
information to resolve it and the template-id fully resolves exactly one 
specialization, we should use that one.  The code in 
resolve_overloaded_unification was trying to do this, but was failing to 
handle the case where there are additional templates that aren't fully 
resolved.

Tested x86_64-pc-linux-gnu, applying to trunk.