Use .into_diagnostic() less.

This commit replaces this pattern:
```
err.into_diagnostic(dcx)
```
with this pattern:
```
dcx.create_err(err)
```
in a lot of places.

It's a little shorter, makes the error level explicit, avoids some
`IntoDiagnostic` imports, and is a necessary prerequisite for the next
commit which will add a `level` arg to `into_diagnostic`.

This requires adding `track_caller` on `create_err` to avoid mucking up
the output of `tests/ui/track-diagnostics/track4.rs`. It probably should
have been there already.

compiler/rustc_errors/src/lib.rs

@@ -1274,6 +1274,7 @@ impl DiagCtxt {
         self.create_err(err).emit()
     }
 
+    #[track_caller]
     pub fn create_err<'a>(
         &'a self,
         err: impl IntoDiagnostic<'a>,