Rollup merge of #120958 - ShoyuVanilla:remove-subst, r=oli-obk

Dejargonize `subst`

In favor of #110793, replace almost every occurence of `subst` and `substitution` from rustc codes, but they still remains in subtrees under `src/tools/` like clippy and test codes (I'd like to replace them after this)

Author: matthiaskrgr