Emit warning when there is no space between -o and confusing arg

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

compiler/rustc_session/src/config.rs

@@ -1653,6 +1653,11 @@ impl RustcOptGroup {
             OptionKind::FlagMulti => options.optflagmulti(short_name, long_name, desc),
         };
     }
+
+    /// This is for diagnostics-only.
+    pub fn long_name(&self) -> &str {
+        self.long_name
+    }
 }
 
 pub fn make_opt(