Follow-up to 14951a5a3120e50084b3c5fb217e2d47992a24d1
* Unify getVariantKindName and getVariantKindForName
* Allow each target to specify the preferred case (albeit ignored in MCParser)
Note: targets that use variant kinds should call MCExpr::print with a
non-null MAI to print variant kinds. operator<< passes a nullptr to
`MCExpr::print`, which should be avoided (e.g. Hexagon; fixed in
commit cf00ac81ac049cddb80aec1d6d88b8fab4f209e8).
98a640a2fa