Sema: implement @errSetCast for error unions

[Vexu]

Closes #17343

[ghost]

Shouldn't it also be renamed to @errorCast? Error sets and error unions are two distinct things aren't they? I think a good way to sum it up is just calling it "error" hence @errorCast (and also not @errCast for consistency with the other builtin names that spell "error" out in full). Or should it be a separate builtin?

[andrewrk]

Follow-up work:

• @errorCast crashes on inferred error set #17353
• returning @errorCast from an inline inferred error set function crashes the compiler #17354
• @errorCast on error unions should allow disjoint sets #17355