scudo: Split setRandomTag in two. NFCI.

Separate the IRG part from the STZG part since we will need to use
the latter on its own for some upcoming changes.

Differential Revision: https://reviews.llvm.org/D92880

GitOrigin-RevId: 9f8aeb0602935f7bb49fb093fedaad5ec4e87497
1 file changed