Investigate and report ICE

> *test-2 and test-3:*
> https://github.com/madmongo1/pfr_review/blob/master/pre-cxx20/test-2.cpp
> Seems to produce infinite recursive template expansion

The example should not compile, because the nested types are not aggregates.

In C++14 it gives a correct static assert

../../../boost/pfr/detail/core14_classic.hpp:509:5: error:
static_assert failed "====================> Boost.PFR: Type can not be
used is flat_ functions, because it's not POD"

in C++17 it also gives a correct static assert

../../../boost/pfr/detail/fields_count.hpp:225:9: error: static
assertion failed: ====================> Boost.PFR: Type must be
aggregate initializable.

Clang crashes after that.
GCC goes into an infinite loop, ignoring all the template
instantiation depth limitations and static_assert messages printing.

Using BOOST_PFR_PRECISE_FUNCTIONS_FOR now gives a complaint of missing
sd::hash specialization:

../../../boost/pfr/detail/functional.hpp:133:9: error: static_assert
failed due to requirement 'sizeof(boost::basic_string_view >) && false' "====================> Boost.PFR:
std::hash not specialized for type T"

I've added the tests and will report issues to the compiler developers soon.