Add more useful error message if clang is missing

hercules_opt/src/pass.rs

@@ -676,7 +676,7 @@ impl PassManager {
                         .stdin(Stdio::piped())
                         .stdout(Stdio::piped())
                         .spawn()
-                        .unwrap();
+                        .expect("Error running clang. Is it installed?");
                     assert!(clang_process.wait().unwrap().success());
                     println!("{}", output_archive);