concreteToGroundVal doesn't handle ConcreteFloats

[RyanGlScott]

As observed in this CI job:

[16 of 36] Compiling SemMC.Util       ( src/SemMC/Util.hs, /runner/_work/macaw/macaw/dist-newstyle/build/x86_64-linux/ghc-8.6.5/semmc-0.1.0.0/build/SemMC/Util.o )

src/SemMC/Util.hs:260:1: warning: [-Wincomplete-patterns]
Warning:     Pattern match(es) are non-exhaustive
    In an equation for ‘concreteToGroundVal’:
        Patterns not matched: (W.ConcreteFloat _ _)
    |
260 | concreteToGroundVal (W.ConcreteBool b) = b
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

ConcreteFloats were introduced in GaloisInc/what4#155.