typeck: clarify def_bm adjustments & add tests for or-patterns #68856
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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 #54883
r? @oli-obk @varkor