Auto merge of rust-lang#146038 - notriddle:polarity, r=GuillaumeGomez

rustdoc-search: split function inverted index by input/output

Fixes rust-lang#146015

With a patch applied to count the number of unifications, and running the query `Option<T>, (T -> U) -> Option<U>`

before: performed unifyFunctionType on 17484 functions
after: performed unifyFunctionType on 3055 functions

preview:
https://notriddle.com/rustdoc-html-demo-12/polarity/doc/std/index.html
https://notriddle.com/rustdoc-html-demo-12/polarity/compiler-doc/rustc_hir/index.html

Author: bors