RFA: PATCH to gimple_canonical_types_compatible_p for middle-end/66214

[Jason Merrill]

The problem here is that we're trying to compare the TYPE_FIELDS of two 
variants of an incomplete type, which doesn't make sense; we shouldn't 
expect TYPE_FIELDS of an incomplete type to be meaningful.

Tested x86_64-pc-linux-gnu.  OK for trunk?
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