Split Handler::emit_diagnostic in two.

Currently, `emit_diagnostic` takes `&mut self`.

This commit changes it so `emit_diagnostic` takes `self` and the new
`emit_diagnostic_without_consuming` function takes `&mut self`.

I find the distinction useful. The former case is much more common, and
avoids a bunch of `mut` and `&mut` occurrences. We can also restrict the
latter with `pub(crate)` which is nice.

Author: nnethercote

src/diagnostics.rs

@@ -512,7 +512,7 @@ pub fn report_msg<'tcx>(
         }
     }
 
-    handler.emit_diagnostic(&mut err);
+    handler.emit_diagnostic(err);
 }
 
 impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {