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/lower/testdata/impl/import_facet.carbon

@@ -36,9 +36,9 @@ interface I {
 library "[[@TEST_NAME]]";
 export import library "b";
 
-class D(T:! type) {
-  impl as I where .T = T* {
-    fn GetI[unused self: Self]() -> C(T*) {
+class D(U:! type) {
+  impl as I where .T = U* {
+    fn GetI[unused self: Self]() -> C(U*) {
       return {};
     }
   }