[NFC] Downgrade The TypeLoc in VarDecl to a TypeRepr
TypeCheckPattern used to splat the interface type into this, and
different parts of the compiler would check one or the other. There is
now one source of truth: The interface type. The type repr is now just
a signal that the user has written an explicit type annotation on
a parameter. For variables, we will eventually be able to just grab
this information from the parent pattern.
