Improve diagnostics and recovery when the nested-name-specifier of a

qualified name does not actually refer into a class/class
template/class template partial specialization. 

Improve printing of nested-name-specifiers to eliminate redudant
qualifiers. Also, make it possible to output a nested-name-specifier
through a DiagnosticBuilder, although there are relatively few places
that will use this leeway.

llvm-svn: 80056