Note that the tests have been updated to initialize the local
variables; originally it was enough just to declare them.
Back when I started this, the `layout_cache` contained entries even
just for types that had been declared but not initialized. Apparently
things have changed in the interim so that if I want one of those
layouts to be computed, I need to actually initialize the value.
(Incidentally, this shows a weakness in the strategy of just walking
the `layout_cache`; the original strategy of using a MIR visitor would
probably have exhibited more robustness in terms of consistent output,
but it had other weaknesses so I chose not to reimplement it. At
least, not yet.)
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Also, I have updated tests to avoid target-specific alignments.
75825fe1df