When demangling a template template parameter (`method<bool,
Bar>(Bar<bool> b)`), the current demangler version first enters the
template argument (`bool`) into the substitutions list, then the whole
template specialization (`Bar<bool>`). The template name (`Bar`) never
becomes a substitution candidate on its own.
This is different when mangling. Mangling `method<bool, Bar>(Bar<bool>
b, Bar<int> i)` substitutes the `Bar` in the second parameter with the
substitution for `TemplateTemplateParmDecl`.
This leads to a discrepancy between mangler and demangler, see
https://github.com/llvm/llvm-project/issues/108009.
2612316f72