Note [ConcreteTv]

A concrete metavariable is a metavariable whose 'MetaInfo' is 'ConcreteTv'.
Similar to 'TyVarTv's which are type variables which can only be unified with
other type variables, a 'ConcreteTv' type variable is a type variable which can
only be unified with a concrete type (in the sense of Note [Concrete types]).

INVARIANT: the kind of a concrete metavariable is concrete.
This invariant is upheld at the time of creation of a new concrete metavariable.

Concrete metavariables are useful for representation-polymorphism checks:
they allow us to refer to a type whose representation is not yet known but will
be figured out by the typechecker (see Note [The Concrete mechanism]).