Hopefully fixing the MinGW 32 build, which was broken by r200767. Not using rand_s() since MinGW does not have an implementation for it, but instead using the underlying CryptGenRandom APIs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201124 91177308-0d34-0410-b5e6-96231b3b80d8