[hwasan] Madvise away unused shadow.
Whenever a large shadow region is tagged to zero, madvise(DONT_NEED)
as much of it as possible.
This reduces shadow RSS on Android by 45% or so, and total memory use
by 2-4%, probably even more on long running multithreaded programs.
CPU time seems to be in the noise.
Reviewers: kcc, pcc
Subscribers: srhines, kubamracek, llvm-commits
Differential Revision: https://reviews.llvm.org/D56757