When template substitution into a template parameter reduces the level

of that parameter, reduce the level by the number of active template
argument lists rather than by 1. The number of active template
argument lists is only > 1 when we have a class template partial
specialization of a member template of a class template that itself is
a member template of another class template. 

... and Boost.MSM does this. Fixes PR7669.

llvm-svn: 112551