C++ PATCH for c++/51930 (instantiation hidden despite visibility attribute)

[Jason Merrill]

While looking at the class variant of this issue, I noticed that some of 
the code in determine_visibility was wrong; template_class_depth only 
considers unbound template parameters, and the number we want is the 
total number of levels.  I've also adjusted the diagnostic for misplaced 
class attributes as manu requested.

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