Fix an ICE when trying to resolve a struct variant #19690
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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.