Auto merge of #51182 - eddyb:not-just-visibility, r=nikomatsakis

[MIR] Change "scopes" from "visibility scopes" to "source scopes".

These scopes are already used for source-oriented diagnostics, lint levels and unsafety checks.
This PR generalizes the naming around scopes, making the type `SourceScope`, and flips (across several commits) the relationship/priority between `LocalDecl`'s "visibility" and "syntactic" scopes.

r? @nikomatsakis