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/function.cpp

@@ -14,7 +14,7 @@ auto CheckFunctionTypeMatches(Context& context,
                               const SemIR::Function& new_function,
                               const SemIR::Function& prev_function,
                               SemIR::SpecificId prev_specific_id,
-                              bool check_syntax) -> bool {
+                              bool check_syntax, bool check_self) -> bool {
   // TODO: When check_syntax is false, the functions should be allowed to have
   // different signatures as long as we can synthesize a suitable thunk. i.e.,
   // when there's an implicit conversion from the original parameter types to
@@ -23,7 +23,7 @@ auto CheckFunctionTypeMatches(Context& context,
   // Also, build that thunk.
   if (!CheckRedeclParamsMatch(context, DeclParams(new_function),
                               DeclParams(prev_function), prev_specific_id,
-                              check_syntax)) {
+                              /*diagnose=*/true, check_syntax, check_self)) {
     return false;
   }