This clears up the printing of a NestedNameSpecifier so a trailing '::'
is not printed, unless it refers into the global scope.
This fixes a bunch of diagnostics where the trailing :: was awkward.
This also prints the NNS quoted consistenty.
There is a drive-by improvement to error recovery, where now we print
the actual type instead of `<dependent type>`.
This will clear up further uses of NNS printing in further patches.
dbd82f33b5