A @_fixed_layout protocol exposes its witness table layout to
clients, which prevents re-ordering of requirements or adding
new requiremenst with a default.
When library evolution is enabled, we still emit method
descriptors even for @_fixed_layout protocols; this allows a
previously-resilient protocol to become @_fixed_layout as long
as the published layout matches the resilient layout in all
previously-shipped versions of the library.
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