* Allows OpTypeForwardPointer to reference IDs not yet declared in
the module
* Allows OpTypeStruct to reference IDs not yet declared in
the module
Possible Issue: OpTypeStruct should only allow forward references
if the ID is a pointer that is referenced by a forward pointer. Need
Type support in Validator which is currently a work in progress.
Every time an event happens in the library that the user should be
aware of, the callback will be invoked.
The existing diagnostic mechanism is hijacked internally by a
callback that creates an diagnostic object each time an event
happens.
Defer removal of a Phi's result id from the undefined-forward-reference
set until after you've scanned the arguments. The reordering is only
significant for Phi.
Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/415
