[CSFix] Add a tailored fix for invalid Swift -> C pointer conversions

This is a skeleton of a fix that would be used to diagnose situations
when Swift -> C pointer conversion was attempted on a Swift function.

lib/Sema/CSSimplify.cpp

@@ -11927,6 +11927,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AddSendableAttribute:
   case FixKind::DropThrowsAttribute:
   case FixKind::DropAsyncAttribute:
+  case FixKind::AllowSwiftToCPointerConversion:
     llvm_unreachable("handled elsewhere");
   }