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/handle_impl.cpp

@@ -254,7 +254,8 @@ static auto MergeImplRedecl(Context& context, SemIR::Impl& new_impl,
   // `impl`. Keep looking for a prior declaration without issuing a diagnostic.
   if (!CheckRedeclParamsMatch(context, DeclParams(new_impl),
                               DeclParams(prev_impl), SemIR::SpecificId::None,
-                              /*check_syntax=*/true, /*diagnose=*/false)) {
+                              /*diagnose=*/false, /*check_syntax=*/true,
+                              /*check_self=*/true)) {
     // NOLINTNEXTLINE(readability-simplify-boolean-expr)
     return false;
   }