Merge #3689

3689: implement fill match arm assist for tuple of enums r=matklad a=JoshMcguigan

This updates the fill match arm assist to work in cases where the user is matching on a tuple of enums.

Note, for now this does not apply when some match arms exist (other than the trivial `_`), but I think this could be added in the future.

I think this also lays the groundwork for filling match arms when matching on tuples of non-enum values, for example a tuple of an enum and a boolean.

Co-authored-by: Josh Mcguigan <joshmcg88@gmail.com>