RustWrapper: adapt for LLVM API change of fatal_error_handler_t

No functional changes intended.

The LLVM commit
e463b69736
changed an argument of fatal_error_handler_t from std::string to char*.
This adapts RustWrapper accordingly.

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -54,7 +54,11 @@ static LLVM_THREAD_LOCAL char *LastError;
 //
 // Notably it exits the process with code 101, unlike LLVM's default of 1.
 static void FatalErrorHandler(void *UserData,
+#if LLVM_VERSION_LT(14, 0)
                               const std::string& Reason,
+#else
+                              const char* Reason,
+#endif
                               bool GenCrashDiag) {
   // Do the same thing that the default error handler does.
   std::cerr << "LLVM ERROR: " << Reason << std::endl;