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_expand/src/base.rs

@@ -1204,11 +1204,10 @@ pub fn resolve_path(
                 .expect("attempting to resolve a file path in an external file"),
             FileName::DocTest(path, _) => path,
             other => {
-                return Err(errors::ResolveRelativePath {
+                return Err(parse_sess.dcx.create_err(errors::ResolveRelativePath {
                     span,
                     path: parse_sess.source_map().filename_for_diagnostics(&other).to_string(),
-                }
-                .into_diagnostic(&parse_sess.dcx));
+                }));
             }
         };
         result.pop();