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/clang_subcommand.cpp

@@ -52,7 +52,7 @@ auto ClangSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
 
   // Don't run Clang when fuzzing, it is known to not be reliable under fuzzing
   // due to many unfixed issues.
-  if (!DisableFuzzingExternalLibraries(driver_env, "clang")) {
+  if (TestAndDiagnoseIfFuzzingExternalLibraries(driver_env, "clang")) {
     return {.success = false};
   }