AMDGPU: Fix broken and permissive handling of printf format strings

This was completely broken with opaque pointers because it was
specifically looking for a constant expression with the global
variable as the first operand. Strip casts like normal, and properly
validate all of the restrictions rather than silently ignoring any
unhandled cases. Also be stricter that we aren't calling into some
unresolved or non-constant format string.

Also converts the test to opaque pointers and generated tests. There's
more broken initializer handling for strings inside the format string
processing too, but there's just no test coverage for this at all.