Rollup merge of #68856 - Centril:or-pat-ref-pat, r=matthewjasper

Rollup merge of #68856 - Centril:or-pat-ref-pat, r=matthewjasper

typeck: clarify def_bm adjustments & add tests for or-patterns

Clarify the adjustment algorithm for the expected type / default binding-modes when type checking patterns with more documentation and tweaks that make the algorithm more independent of the pattern forms.

Also resolve the FIXME noted for or-patterns by deciding that the current implementation is correct, noting the rationale and adding tests for the current implementation.

cc https://github.com/rust-lang/rust/issues/54883

r? @oli-obk @varkor