Auto merge of #110662 - bryangarza:safe-transmute-reference-types, r=compiler-errors

Safe Transmute: Enable handling references

This patch enables support for references in Safe Transmute, by generating nested obligations during trait selection. Specifically, when we call `confirm_transmutability_candidate(...)`, we now recursively traverse the `rustc_transmute::Answer` tree and create obligations for all the `Answer` variants, some of which include multiple nested `Answer`s.

Author: bors