[GWP-ASan] Add thread ID to PRNG seed.
Adds thread ID to PRNG seed for increased entropy. In particular, this allows
multiple runs in quick succession that will have different PRNG seeds, allowing
for better demos/testing.
Reviewed By: kcc
Subscribers: kubamracek, #sanitizers, llvm-commits
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D64453