This commit fixes issue #1631. However, it is not a perfect solution.
This commit improves the predicate that checks whether a definition is
noncomputable or not. This predicate was implemented before we had
a code generator.
We should refactor the code and use the code generator to check
whether a definition is noncomputable or not. Otherwise, we will
keep finding mismatches between the predicate at noncomputable.cpp
and what the code generator implements.
Now, eta and impredicativity are assumed to be always true.
Motivation: the rest of the system assumes eta.
Regarding impredicativity, we decided to support only the standard library.
