10704: internal: Short-circuit `descend_into_macros_single` r=Veykril a=Veykril
There is no need to descend everything if all we are interested in is the first mapping.
This bring `descend_into_macros` timing in highlighting in `rust-analyzer/src/config.rs` from `154ms - descend_into_macros (2190 calls)` to `24ms - descend_into_macros (2190 calls)` since we use the single variant there(will regress once we want to highlight multiple namespaces again though).
bors r+
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
726b4dd8bd