Add error for virtual member function without self (#5005)

This tripped over a lowering crash when a member function with self was
declared-but-not-defined, so that's why some test cases were updated to
have (empty) function definitions.

I'll follow-up with/look into a fix for the
self-declared-but-not-defined cases separately.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>

toolchain/check/impl.cpp

@@ -72,8 +72,8 @@ static auto CheckAssociatedFunctionImplementation(
   if (!CheckFunctionTypeMatches(
           context, context.functions().Get(impl_function_decl->function_id),
           context.functions().Get(interface_function_type.function_id),
-          interface_function_specific_id,
-          /*check_syntax=*/false)) {
+          interface_function_specific_id, /*check_syntax=*/false,
+          /*check_self=*/true)) {
     return SemIR::ErrorInst::SingletonInstId;
   }
   return impl_decl_id;