As part of this, we have to change the type export rules to
prevent `@convention(c)` function types from being used in
exported interfaces if they aren't serializable. This is a
more conservative version of the original rule I had, which
was to import such function-pointer types as opaque pointers.
That rule would've completely prevented importing function-pointer
types defined in bridging headers and so simply doesn't work,
so we're left trying to catch the unsupportable cases
retroactively. This has the unfortunate consequence that we
can't necessarily serialize the internal state of the compiler,
but that was already true due to normal type uses of aggregate
types from bridging headers; if we can teach the compiler to
reliably serialize such types, we should be able to use the
same mechanisms for function types.
This PR doesn't flip the switch to use Clang function types
by default, so many of the clang-function-type-serialization
FIXMEs are still in place.
