js: support variant destructors

[zerbina]

Summary

Fix destructors not being properly executed for fields within case
objects when changing the active branch, and using the JS backend.

Details

The destructor produced by produceDestructorForDiscriminator took a
reference to the discriminant field and used pointer arithmetic to
compute the address of the enclosing object. Not only is this
unnecessarily complex, it also doesn't work with the JS backend.

Instead, the destructor now takes a mutable reference to the
discriminant field's enclosing object type -- the MIR emitted for the
call is adjusted accordingly.

Since the destructor definitely modifies the object, ekMutate is used
instead of ekInvalidate for the parameter passing. This results in a
small move analyzer regression:

var x = Object()
var y = x.fieldOutsideOfVariant # not automatically moved anymore
x.kind = ... # potential branch switch that requires destruction

Finally, a test is added for ensuring proper destruction when changing
the active branch.

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• fixing the regression requires moving the mnkSwitch lowering to a separate pass, which is then run after the move-optimization pass
• fixing the test for the VM is doable, but requires some changes to the lowerBranchSwitch pass