Change branch-to-fatal to check condition (#4256)

toolchain/lower/file_context.cpp

@@ -242,13 +242,12 @@ auto FileContext::BuildFunctionDecl(SemIR::FunctionId function_id)
   if (SemIR::IsEntryPoint(sem_ir(), function_id)) {
     // TODO: Add an implicit `return 0` if `Run` doesn't return `i32`.
     mangled_name = "main";
-  } else if (auto name =
-                 sem_ir().names().GetAsStringIfIdentifier(function.name_id)) {
+  } else {
     // TODO: Decide on a name mangling scheme.
+    auto name = sem_ir().names().GetAsStringIfIdentifier(function.name_id);
+    CARBON_CHECK(name) << "Unexpected special name for function: "
+                       << function.name_id;
     mangled_name = *name;
-  } else {
-    CARBON_FATAL() << "Unexpected special name for function: "
-                   << function.name_id;
   }
 
   llvm::FunctionType* function_type = llvm::FunctionType::get(