[clang][bytecode] Allocate InitMaps via Program/InterpState allocators (#170272)
Save them as a pointer intead of using a shared_ptr. This we we can use
the pointer integer value to differentiate the "no initmap yet" and "all
values initialzed" cases.
This regresses one test case in const-eval.c, but as it turns out, that
only worked coincidentally before.