Itanium ABI: Improve our mangling of <destructor-name>

Our mangling of <destructor-name> wasn't quite right: we'd introduce
mangling substitutions where one shouldn't be possible.  We also didn't
correctly handle the case where the destroyed type was not dependent but
still a TemplateSpecializationType.

N.B. There isn't a mangling for a template-template parameter showing up
as the destroyed type.  We do the 'obvious' thing and mangle the index
of the parameter.

llvm-svn: 229615