Unlike a tuple variant constructor which can be called as a function, a
struct variant constructor is not a function, so cannot be called.
If the user tries to assign the constructor to a variable, an ICE
occurs, because there is no way to use it later. So we should stop the
constructor from being used like that.
A similar mechanism already exists for a normal struct, as it prohibits
a struct from being resolved. This commit does the same for a struct
variant.
This commit also includes some changes to the existing tests.
Fixes#19452.
