tsan: fix windows support

UnmapOrDie used to do MEM_DECOMMIT and so worked
on partial regions. But r263160 changed it to use
MEM_RELEASE and MEM_RELEASE can only work with
whole regions mapped by VirtualAlloc. This broke
windows as:

FATAL: ThreadSanitizer CHECK failed: gotsan.cc:8296 "((mbi.AllocationBase == addr && "Windows cannot unmap part of a previous mapping")) != (0)" (0x0, 0x0)

Restore the previous behavior.

llvm-svn: 267730