Adaps the transformations that add OpConstantUndef and OpConstantNull
to a module so that pointer undefs are not allowed, and null pointers
are only allowed if suitable capabilities are present.
Fixes#4357.
Adds a virtual method, GetFreshIds(), to Transformation. Every
transformation uses this to indicate which ids in its protobuf message
are fresh ids. This means that when replaying a sequence of
transformations the replayer can obtain a smallest id that is not in
use by the module already and that will not be used by any
transformation by necessity. Ids greater than or equal to this id
can be used as overflow ids.
Fixes#3851.
