Rollup merge of rust-lang#65379 - Centril:refactor-bso_and_cc, r=petrochenkov

refactor session::config::build_session_options_and_crate_config

I'm not exactly sure what happened in rust-lang#65361 (comment) so in an effort to bisect the error I'm extracting out the first commit so it can hopefully land (I suspect the error is in the 2nd-3rd commits.)

r? @petrochenkov

(I'll r=you when the PR builder is happy)

Author: Centril