The key_type type definition for map containers is useful in some generic, template-based programming scenarios. The addition of key_type to MapVector is consistent with other map types like DenseMap.
The only way I know is re-uploading the diff.
I tried simply restarting the build but it didn't include the fix.
If you upload with arcanist it also stores the parent git SHA1 and would just re-apply the patch to that. If you upload via web form Phabricator does not have parent information, but I don't know what happens when triggering a re-build. Might either use the opt-of-tree at time of uploading the diff or the top-of-tree when running the pre-merge check.