This came up because `const T` needs destructor support... This change
makes `impl T as Destroy` and `impl const T as Destroy` distinct type
structures. Right now there's no impl lookup fallback (see
[#6068](https://github.com/carbon-language/carbon-lang/issues/6068)); so
when trying to destroy `const T`, there's no way to have an `impl` for
it to find.
In discussion, `MaybeUnformed` and `partial` have similar challenges, so
I'm covering them together.
In type_structure.h, I'm switching to an enum because it felt like an
easier way to be adding more types. I can switch back if preferred,
though then might take a closer look at the `operator==` because that's
kind of verbose.
Jon Ross-Perkins
59c4cbcaf1