Mark all extern statics FIXED, covering both static_tys[def_id] and addr_of_static[def_id]
Don't attempt to change static to static mut or vice versa if its addr_of_static is FIXED
Together, the last two prevent attempts to rewrite extern statics, which would otherwise panic because gen_static_rewrites isn't designed to handle extern statics (it expects the DefId to refer to a non-extern HIR item).
This has three parts:
gacx.static_tysFIXED, covering bothstatic_tys[def_id]andaddr_of_static[def_id]statictostatic mutor vice versa if itsaddr_of_staticis FIXEDTogether, the last two prevent attempts to rewrite extern statics, which would otherwise panic because
gen_static_rewritesisn't designed to handle extern statics (it expects theDefIdto refer to a non-extern HIR item).