Auto merge of #47167 - ivanbakel:builtin_indexing, r=nikomatsakis

Fix built-in indexing not being used where index type wasn't "obviously" usize

Fixes #33903
Fixes #46095

This PR was made possible thanks to the generous help of @eddyb

Following the example of binary operators, builtin checking for indexing has been moved from the typecheck stage to a writeback stage, after type constraints have been resolved.

diff --cc src/librustc_typeck/check/mod.rs
Simple merge