Fix an ICE when trying to resolve a struct variant

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.
auto merge of #19690 : barosl/rust/struct-variant-as-a-function-ice, r=alexcrichton

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.