stage2: fix switch on tagged union capture-by-pointer

 * AstGen: always use `typeof` and never `typeof_elem` on the
   `switch_cond`/`switch_cond_ref` instruction because both variants
   return a value and not a pointer.
   - Delete the `typeof_elem` ZIR instruction since it is no longer
     needed.
 * Sema: validateUnionInit now recognizes a comptime mutable value and
   no longer emits a compile error saying "cannot evaluate constant
   expression"
   - Still to-do is detecting comptime union values in a function that
     is not being executed at compile-time.
     - This is still to-do for structs too.
 * Sema: when emitting a call AIR instruction, call resolveTypeLayout on
   all the parameter types as well as the return type.
 * `Type.structFieldOffset` now works for unions in addition to structs.

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