The main point of this change is to make sure that a shared function always has a body: both, in the optimizer pipeline and in the swiftmodule file.
This is important because the compiler always needs to emit code for a shared function. Shared functions cannot be referenced from outside the module.
In several corner cases we missed to maintain this invariant which resulted in unresolved-symbol linker errors.
As side-effect of this change we can drop the shared_external SIL linkage and the IsSerializable flag, which simplifies the serialization and linkage concept.
The existence of a shared_external function in itself is not
an error; it just means we deserialized a witness table or
vtable but did not need to deserialize a thunk.
However, a direct reference to such a function is an error,
because we should have deserialized the body in that case.
This fixes a crasher, but the SIL crashers are kind of silly
because the SIL parser does not try at all not to crash on
invalid input.
