The virtual-dispatch code for globals was missing any relationship
between the union field access and the global variable, which meant it
propagated function-pointer flow between any two fields of a global
struct. This resulted in false positives from
`cpp/tainted-format-string` on projects using SDL, such as
WohlSoft/PGE-Project.
In addition to fixing that bug, this commit also brings the code up to
date with the new style of modeling flow through global variables:
`DataFlow::Node.asVariable()`.
76e194c8be