Mangling for C++ pack indexing

[cor3ntin]

Hey folks

Pack indexing (https://wg21.link/p2662r3) was approved for C++26.
We will need to mangle these things.

A pack indexing is constituted of a pattern (which is either a type or an expression) and an indexing expression.
I am unfortunately not familiar enough with the ABI to offer suggestions

[rjmccall]

Looks like it's a new type-level operator and a new expression-level operator. Apparently it will eventually be accepted as a template-name as well, but that's not currently included? Anyway, this should be straightforward; we just need to add new forms to <expression> and <type>.

Typical uses of this are probably going to exacerbate our existing problems with partial substitution, though. Consider when a pack indexing operator is used in the signature of a templated member of a class template and we can't expand the pack in the instantiated dependent signature of the template:

template <class... T> struct tuple {
  template <unsigned I> T...[I] get();
};

We'll need to solve that problem.