[sanitizer] Revert rL303879 as it breaks Windows

Summary:
Apparently Windows's `UnmapOrDie` doesn't support partial unmapping. Which
makes the new region allocation technique not Windows compliant.

Reviewers: alekseyshl, dvyukov

Reviewed By: alekseyshl

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D33554

llvm-svn: 303883