[hwasan] Madvise away unused shadow.

Summary:
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

llvm-svn: 351620