swift234

SILGen: Add asserts that bridged types are loadable

Bridging thunks don't yet support bridging address-only types, but
ideally they should, so that we can bridge Objective-C types to
resilient value types.

This came up while I was adding @_fixed_layout declarations in
the standard library. To make this easier to figure out in the
future, add the asserts to the bridging logic for now.

Also, to avoid hitting the asserts when we emit a reference to a
C function with an incompatible type, don't emit the foreign
function at all until we determine that the ABI conversion is safe.