Rollup merge of #148865 - lcnr:gat-inference-hack, r=BoxyUwU

move GAT inference prevention hack

The structure of `fn assemble_and_merge_candidates` is quite messy and the differences between `Host` and `NormalizesTo` goals is large enough that we should split them entirely. Intend to change this for rust-lang/trait-system-refactor-initiative#245 by mentoring someone: https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/ask.20for.20help/near/554696331. Think it's still fine to merge this PR without that larger change.

fixes rust-lang/trait-system-refactor-initiative#256

r? `@BoxyUwU`

Author: matthiaskrgr