Support accessing associated functions by member access into facets (#4872)

For an expression such as `(Type as Interface).AssocFn()`, track the
`Self` type `Type` in the result of the member access so that it's
available when checking the function call.

This introduces a new kind of type, `ImplFunctionType`, that represents
the type of a function that is expected within an impl, modeled as the
type of the function within the interface plus a value to use as `Self`.
Calls to values of this type behave like calls to the underlying
function except that the `Self` parameter is pre-bound to the self type
from the facet.

In order to support this, fix an issue where the imported list of
generic bindings lost their association with their enclosing generic.
This adds a little complexity to `import_ref`, including a new recursive
cycle that I intend to address in a follow-up PR.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/context.cpp

@@ -1292,7 +1292,8 @@ class TypeCompleter {
   template <typename InstT>
     requires(InstT::Kind.template IsAnyOf<
              SemIR::AssociatedEntityType, SemIR::FacetAccessType,
-             SemIR::FacetType, SemIR::FunctionType, SemIR::GenericClassType,
+             SemIR::FacetType, SemIR::FunctionType,
+             SemIR::FunctionTypeWithSelfType, SemIR::GenericClassType,
              SemIR::GenericInterfaceType, SemIR::UnboundElementType,
              SemIR::WhereExpr>())
   auto BuildValueReprForInst(SemIR::TypeId /*type_id*/, InstT /*inst*/) const
@@ -1555,6 +1556,13 @@ auto Context::GetFunctionType(SemIR::FunctionId fn_id,
   return GetCompleteTypeImpl<SemIR::FunctionType>(*this, fn_id, specific_id);
 }
 
+auto Context::GetFunctionTypeWithSelfType(
+    SemIR::InstId interface_function_type_id, SemIR::InstId self_id)
+    -> SemIR::TypeId {
+  return GetCompleteTypeImpl<SemIR::FunctionTypeWithSelfType>(
+      *this, interface_function_type_id, self_id);
+}
+
 auto Context::GetGenericClassType(SemIR::ClassId class_id,
                                   SemIR::SpecificId enclosing_specific_id)
     -> SemIR::TypeId {