[Assignment Tracking] Fix -fexperimental-assignment-tracking cc1 flag

This change allows `-fexperimental-assignment-tracking` to be negated using the
negative equivalent `-fno-experimental-assignment-tracking`, and fixes the flag
so that the default behaviour can be changed (currently the default is still
false / disabled).

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D142029