Support unqualified lookup into extended interfaces from classes and impls (#7217)

When a class extends an interface, referring to a member name of the
interface as an unqualified name should refer to the class's
corresponding associated entity value, not to the associated entity
itself. Similarly, in an `impl`, unqualified names of associated
entities should refer to the `impl`'s corresponding value for that
entity.

To support this, we treat `impl`s as `extend`ing their implemented facet
type, and we make lookups into an extended facet type use the `Self`
type of the extending `impl` or `class` if lookup finds an associated
entity. We already did the latter if the extending entity was an
interface; this extends the existing support for these other cases.

toolchain/check/testdata/function/declaration/name_poisoning.carbon

@@ -95,12 +95,7 @@ alias N.F2 = F1;
 // CHECK:STDERR:
 fn N.F1();
 
-// Failure: `N.F1` used after declaration failed.
-// TODO: #4622 - Allow defining a poisoned name so it would be found if used after it's declared.
-// CHECK:STDERR: fail_use_declaration_after_poison.carbon:[[@LINE+4]]:14: error: member name `F1` not found in `N` [MemberNameNotFoundInInstScope]
-// CHECK:STDERR: alias N.F3 = N.F1;
-// CHECK:STDERR:              ^~~~
-// CHECK:STDERR:
+// Don't produce an additional diagnostic here.
 alias N.F3 = N.F1;
 
 // --- fail_poison_multiple_scopes.carbon