Looks fine to me, but Timur is the expert here.
================
Comment at: lib/asan/CMakeLists.txt:109
@@ +108,3 @@
+
+    if (WIN32)
+      add_compiler_rt_static_runtime(clang_rt.asan_dll_thunk-${arch} ${arch}
----------------
Yep, makes sense to be win32 here instead of msvc.
http://llvm-reviews.chandlerc.com/D1525