[PATCH] Enable Asan on Windows

[Timur Iskhodzhanov]

[Sorry I forgot to mention - please add llvm-commits@ to subscribers for all your compiler-rt patches]

The patch looks mostly fine for me, I'll refine it slightly and commit soon (when I have some extra time).

Can you please confirm that this patch doesn't regress "ninja check-all" in your build environment?
(i.e. it's fine if most things that were broken remain broken; it's not OK if anything else starts to fail)

http://reviews.llvm.org/D5367