Add an extra check in kcCheckDeclHeader_sig

Fix #24083 by checking for a implicitly-scoped type variable that is not
actually bound.  See Note [Disconnected type variables] in GHC.Tc.Gen.HsType

For some reason, on aarch64-darwin we saw a 2.8% decrease in compiler
allocations for MultiLayerModulesTH_Make; but 0.0% on other architectures.

Metric Decrease:
    MultiLayerModulesTH_Make