Do not require that function sorts are first class internally (#4128)

Do not require that function sorts are first class internally (#4128)

This PR removes the requirement in the NodeManager that argument types to the function sort are first class.

Notice that the new API does this check (as it should): https://github.com/CVC4/CVC4/blob/master/src/api/cvc4cpp.cpp#L2633

Moreover, SyGuS v2 internally requires constructing function types having arguments that are not first class (e.g. regular expression type). This is required to update the regression https://github.com/CVC4/CVC4/blob/master/test/regress/regress1/sygus/re-concat.sy to SyGuS v2.

FYI @abdoo8080 .