We have had a heuristic that lets you reflect fields of types, where
the field's type is conditionally Copyable, despite the reflection
infrastructure always copying a field. That heuristic was added to allow
ubiquitous types like Optional in the stdlib to continue to be
reflected, even if the Optional at runtime really isn't Copyable.
As a consequence, we were also allowing reflection of fields containing
user-defined conditionally copyable types, when that's unsafe for no
real benefit, yielding runtime crashes!
I think in that case it's better to fall-back on the non-crashy case of
reflection seeing it as the EmptyTupleType, which isn't inhabited, so it
won't try to copy the field and instead basically skip-over it until a
future runtime supports the reflection safely.
So, this patch limits the dangerous reflection to only stdlib-defined
types, until Mirror and friends are updated.
