Implement __stosb intrinsic as a volatile memset

Implement __stosb intrinsic as a volatile memset

Summary: We need `__stosb` to be an intrinsic, because SecureZeroMemory function uses it without including intrin.h. Implementing it as a volatile memset is not consistent with MSDN specification, but it gives us target-independent IR while keeping the most important properties of `__stosb`.

Reviewers: rnk, hans, thakis, majnemer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25334

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@284253 91177308-0d34-0410-b5e6-96231b3b80d8