Factor out fuzzing disablement in the driver (#5048)

We end up needing to do this in any driver subcommand that reaches into
external code that may not be fully fuzz-clean. No need to grow multiple
different diagnostics for each, we can use a common diagnostic.

toolchain/driver/driver_subcommand.cpp

@@ -0,0 +1,29 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/driver/driver_subcommand.h"
+
+#include "llvm/TargetParser/Host.h"
+#include "llvm/TargetParser/Triple.h"
+#include "toolchain/driver/lld_runner.h"
+
+namespace Carbon {
+
+auto DriverSubcommand::DisableFuzzingExternalLibraries(DriverEnv& driver_env,
+                                                       llvm::StringRef name)
+    -> bool {
+  // Only need to do anything when fuzzing.
+  if (!driver_env.fuzzing) {
+    return true;
+  }
+
+  CARBON_DIAGNOSTIC(
+      ToolFuzzingDisallowed, Error,
+      "preventing fuzzing of `{0}` subcommand due to external library",
+      std::string);
+  driver_env.emitter.Emit(ToolFuzzingDisallowed, name.str());
+  return false;
+}
+
+}  // namespace Carbon