I don't recall why I put these checks here -- they aren't correct. We
can freely recreate a type even if its fields have changed, because we
are going to re-do all type resolution.
The only conditions for recreations are (a) the ZIR index must not be
lost and (b) the number of captures must be the same. These conditions
are permissible because if either is violated, we can guarantee that
analysis of a valid `zirStructDecl` (etc) will never reference this
type (since the ZIR index has just been tracked, and the captures have
just been created based on the ZIR).
Adds a corresponding test case.
Resolves: #21185
