This change adds a new kind of fact to the fact manager, which records
when a variable (or pointer parameter) refers to an arbitrary value,
so that anything can be stored to it, without affecting the observable
behaviour of the module, and nothing can be guaranteed about values
loaded from it. Donated modules are the current source of such
variables, and other transformations, such as outlining, have been
adapted to propagate these facts appropriately.
In the context of SPIR-V 1.4 or higher, global variables cannot be
used by an instruction unless they are listed in the interface of all
entry points that might invoke the instruction. This change
conservatively adds new global variables to the interfaces of all
entry points (if the SPIR-V version is 1.4 or higher).
Issue #3111 notes that a more rigorous approach to entry point
interfaces could be taken in spirv-fuzz, which would allow being less
conservative here.
