Use the right level with -Ztreat-err-as-bug.

Errors in `DiagCtxtInner::emit_diagnostic` are never set to
`Level::Bug`, because the condition never succeeds, because
`self.treat_err_as_bug()` is called *before* the error counts are
incremented.

This commit switches to `self.treat_next_err_as_bug()`, fixing the
problem. This changes the error message output to actually say "internal
compiler error".

compiler/rustc_errors/src/lib.rs

@@ -1256,7 +1256,7 @@ impl DiagCtxtInner {
     }
 
     fn emit_diagnostic(&mut self, mut diagnostic: Diagnostic) -> Option<ErrorGuaranteed> {
-        if matches!(diagnostic.level, Error | Fatal) && self.treat_err_as_bug() {
+        if matches!(diagnostic.level, Error | Fatal) && self.treat_next_err_as_bug() {
             diagnostic.level = Bug;
         }