Support associated type defaults

[Veykril]

We currently fail to fallback to them if the impl misses them which causes problems with rust-lang/rust's query system. cc https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/rustc.20query.20types.20are.20not.20normalized.20since.20recently

[WaffleLapkin]

FYI: we've removed the use of associated type defaults from the compiler query engine in rust-lang/rust#105493, as a temporary fix. It would still be great if r-a could just™ support associated type defaults though.