Why isn't `BasicBlock` autofilled for each type?

[iwinux]

Currently we need to explicitly specify BasicBlock, set the Type field with consts like BlockTypeTableOfContents. It's very tedious and error-prone (no validation if I set the wrong type for TableOfContentsBlock.BasicBlock.Type).

[amberpixels]

I had such helper constructors in my local fork, so it's been requested - I'm making a PR