swift234

[Name lookup] Use the overload signature's type for shadowing of var/subscript decls.

For historical reasons, the "name shadowing" computation is only
looking at the type---not even the interface type!---of
declarations. For variables and subscripts, this means that the
context (e.g., a constrained extension) wasn't been considered at all,
leading to declarations from other imported modules being
ignored. Patch up a little bit of this by using the overload
signature's type for variables and subscripts, because I need it for
ErrorProtocol's default implementations.

Longer-term, we should be using the overload signature (or something
very like it) for shadowing, consistently.