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>
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