Auto merge of #17908 - ChayimFriedman2:usages-word-boundaries, r=Veykril

Test for word boundary in `FindUsages`

This speeds up short identifiers search significantly, while unlikely to have an effect on long identifiers (the analysis takes much longer than some character comparison).

Tested by finding all references to `eq()` (from `PartialEq`) in the rust-analyzer repo. Total time went down from 100s to 10s (a 10x reduction!).

Feel free to close this if you consider this a non-issue, as most short identifiers are local.

Author: bors