[embedded] Fix IR verification crash when using arrays of zero-sized structs

[kubamracek]

The embedded Swift custom ("no runtime calls") IRGen for array builtins today doesn't handle zero-sized structs, when using an array of zero-sized structs, LLVM IR verifier complains:

GEP into unsized type!
  %8 = getelementptr inbounds %T4main8MyStructV, ptr %1, i64 %6

Let's fix that and handle zero-sized structs in GenBuiltin (we don't really need to emit the loop at all for them).

[kubamracek]

@swift-ci please test

[eeckstein]

Do you know why this isn't a problem in regular swift, too?

[kubamracek]

I think that's because in destroy we either do not (for POD) or always emit a runtime call (and the runtime implementation handles zero size correctly):

void TypeInfo::destroyArray(IRGenFunction &IGF, Address array,
                            llvm::Value *count, SILType T) const {
  if (isTriviallyDestroyable(ResilienceExpansion::Maximal))
    return;

 emitDestroyArrayCall(IGF, T, array, count);
}

And same for copy/init/take/assign -- it's either a memcpy/memmove or a runtime call:

void TypeInfo::assignArrayWithTake(IRGenFunction &IGF, Address dest,
                                              Address src, llvm::Value *count,
                                              SILType T) const {
  if (isTriviallyDestroyable(ResilienceExpansion::Maximal)) {
    llvm::Value *stride = getStride(IGF, T);
    llvm::Value *byteCount = IGF.Builder.CreateNUWMul(stride, count);
    IGF.Builder.CreateMemCpy(
        dest.getAddress(), llvm::MaybeAlign(dest.getAlignment().getValue()),
        src.getAddress(), llvm::MaybeAlign(src.getAlignment().getValue()),
        byteCount);
    return;
  }

  emitAssignArrayWithTakeCall(IGF, T, dest, src, count);
}

So we never GEP into the struct in regular Swift.

[kubamracek]

@swift-ci please test macOS platform

[eeckstein]

lgtm!