Tweak the name for the function that diagnoses when fuzzing external libraries (#5974)

The old function name caused some confusion during the review of #5338,
sending this to see if it provides a less surprising function name and
boolean result. Happy to try other names / approaches as well.

toolchain/driver/llvm_subcommand.cpp

@@ -63,7 +63,7 @@ auto LLVMSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
   LLVMRunner runner(driver_env.installation, driver_env.vlog_stream);
 
   // Don't run arbitrary LLVM tools and libraries when fuzzing.
-  if (!DisableFuzzingExternalLibraries(driver_env, "llvm")) {
+  if (TestAndDiagnoseIfFuzzingExternalLibraries(driver_env, "llvm")) {
     return {.success = false};
   }