Auto merge of rust-lang#63446 - Centril:refactor-lowering, r=oli-obk

Refactor and categorize lowering wrt. items / exprs

Split lowering into more files along the lines of "expression related" and "item related".
Also refactor huge methods into smaller ones.

A next step might be to introduce "type related" and merge patterns and statements combined with expressions into "value related". There's still more work to do but the PR was getting too big :)

r? @oli-obk