Fix issue in datatypes care graph computation involving subtyping (#8125)

Fix issue in datatypes care graph computation involving subtyping (#8125)

The datatypes care graph was incorrectly computed for constructors taking `Real` arguments, due to subtyping.

This fixes the term index to cache on the *return* type for constructors, not the argument types.  Caching based on argument types was incorrect even for non-parametric datatypes with arguments that have subtyping.

Fixes https://github.com/cvc5/cvc5/issues/8124.

This also does minor cleanup and additions done while debugging the issue.