Rollup merge of #87200 - oli-obk:fixup_fixup_opaque_types, r=nikomatsakis

TAIT: Infer all inference variables in opaque type substitutions via InferCx

The previous algorithm was correct for the example given in its
documentation, but when the TAIT was declared as a free item
instead of an associated item, the generic parameters were the
wrong ones.

cc `@spastorino`

r? `@nikomatsakis`

diff --cc compiler/rustc_middle/src/ty/mod.rs
Simple merge