Suggest non-ambiguous comparison after cast

```
warning: `<` is interpreted as a start of generic arguments for `usize`, not comparison
  --> $DIR/issue-22644.rs:16:33
   |
16 |     println!("{}", a as usize < b);
   |                                 ^ expected one of `!`, `(`, `+`, `,`, `::`, or `>` here
   |
help: if you want to compare the casted value then write
   |     println!("{}", (a as usize) < b);
```

1 files changed, 4 insertions, 0 deletions

diff --git a/src/librustc_errors/diagnostic.rs b/src/librustc_errors/diagnostic.rs
index 7a64cdeee65..d7c21127474 100644
--- a/src/librustc_errors/diagnostic.rs
+++ b/src/librustc_errors/diagnostic.rs
@@ -248,6 +248,10 @@ impl Diagnostic {
         self.message.iter().map(|i| i.0.to_owned()).collect::<String>()
     }
 
+    pub fn set_message(&mut self, message: &str) {
+        self.message = vec![(message.to_owned(), Style::NoStyle)];
+    }
+
     pub fn styled_message(&self) -> &Vec<(String, Style)> {
         &self.message
     }