Recommit "[SCCP] Remove forcedconstant, go to overdefined instead"

This includes a fix for cases where things get marked as overdefined in
ResolvedUndefsIn, but we later discover a constant. To avoid crashing,
we consistently bail out on overdefined values in the visitors. This is
similar to the previous behavior with forcedconstant.

This reverts the revert commit 02b72f56.