[ASan/Win] Disable warning C4391: incorrect return type for intrinsic function

This warning makes it a bit hard to define strlen and wcslen interceptors in an elegant yet portable way

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@216450 91177308-0d34-0410-b5e6-96231b3b80d8