Rollup merge of #37848 - nnethercote:UnificationTable-probe, r=arielb1
Rollup merge of #37848 - nnethercote:UnificationTable-probe, r=arielb1

Don't clone in UnificationTable::probe().

This speeds up compilation of rustc-benchmarks/inflate-0.1.0 by 1%.

aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust