Rollup merge of #48181 - michaelwoerister:fix-incr-dir-finalization, r=nikomatsakis

Rollup merge of #48181 - michaelwoerister:fix-incr-dir-finalization, r=nikomatsakis

incr.comp.: Run cache directory garbage collection before loading dep-graph.

Prior to this PR, the incr. comp. cache directory would only be garbage collected after the final output artifacts were generated. However, compilation often aborts earlier and in the case of the RLS, which starts lots of compilation sessions, we might fill up the cache directory with chunk sessions.

This PR makes the compiler do a garbage collection run before loading the dep-graph.

cc @nrc https://github.com/rust-lang/rust/issues/48172

r? @nikomatsakis