[ASan/Win] Refactor __interception::OverrideFunction a bit, allow the orig_old_func parameter to be zero

Reviewed at http://reviews.llvm.org/D3798

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@208989 91177308-0d34-0410-b5e6-96231b3b80d8