Primarily this means that with Clang we use `__builtin_debugtrap`
if available.

Break specific purpose generators into random and range generator sections.

This stops tests failing falsely if the assertion passed, but the
stringification itself failed as the assertion was sent to the reporter.

I don't think that stringification should be fallible, but the
overhead in compilation ended up being small enough (<0.5% on `SelfTest`)
that it might be worth implementing, in case there is more users
with weird `StringMaker`s than just MongoDB.

Closes #2980

Fixes #2685

As it turns out, enums can be used to declare bitfields, and we
cannot form a reference to a bitfield in the cpature. Thus, we add
`std::is_enum` as a criteria to the default for `capture_by_value`,
so that enum-based bitfields are also captured by value and thus
decomposable.

Closes #3001

Having `std::terminate` as the backstop after `__assume(false)`
would trigger W4702 (unreachable code) with MSVC. As we want to
keep `__assume(false)` for the optimization hint in NDEBUG builds,
we have to avoid mixing in `std::terminate` for those builds.

Fixes #3007