Make errors::Handler::cancel() take an immutable reference.

This allows to cancel an error with only an immutable access to the `Handler`. Mutable access is already not required to emit an error anyway.

Author: LeoTestard

src/libsyntax/errors/mod.rs

@@ -496,7 +496,7 @@ impl Handler {
         DiagnosticBuilder::new(self, Level::Fatal, msg)
     }
 
-    pub fn cancel(&mut self, err: &mut DiagnosticBuilder) {
+    pub fn cancel(&self, err: &mut DiagnosticBuilder) {
         if err.level == Level::Error || err.level == Level::Fatal {
             assert!(self.has_errors());
             self.err_count.set(self.err_count.get() - 1);