PrimTyCon.primRepName uses Maybe no for good reason

Currently, PrimTyCon's primRepName field has type Maybe TyConRepName. But there is no good reason for this, as in every place where a PrimTyCon is constructed, primRepName will be instantiated with a Just. I propose changing primRepName to be of type TyConRepName instead to simplify this. What's more, this won't even change the exported API in GHC.Core.TyCon, as the fields of PrimTyCon aren't exported.

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