[scudo][standalone] Restore GWP-ASan flag parsing
With D92696, the Scudo Standalone GWP-ASan flag parsing was changed to
the new GWP-ASan optional one. We do not necessarily want this, as this
duplicates flag parsing code in Scudo Standalone when using the
This CL reverts the changes within Scudo Standalone, and increases
MaxFlags to 20 as an addionnal option got us to the current max.
Differential Revision: https://reviews.llvm.org/D95542