Auto merge of #119947 - compiler-errors:old-solver-instantiate-response, r=lcnr

Make sure to instantiate placeholders correctly in old solver

When creating the query substitution guess for an input placeholder type like `!1_T` (in universe 1), we were guessing the response substitution with something like `!0_T`. This failed to unify with `!1_T`, causing an ICE.

This PR reworks the query substitution guess code to work a bit more like the new solver. I'm *pretty* sure this is correct, though I'd really appreciate some scrutiny from someone (*cough* lcnr) who knows a bit more about query instantiation :)

Fixes #119941

r? lcnr

Author: bors