[clang] Add support for __attribute__((guard(nocf)))

To support using Control Flow Guard with mingw-w64, Clang needs to
accept `__declspec(guard(nocf))` also for the GNU target. Since mingw
has `#define __declspec(a) __attribute__((a))` as built-in, the simplest
solution is to accept `__attribute__((guard(nocf)))` to be compatible with
MSVC and Clang's msvc target.

As a side effect, this also adds `[[clang::guard(nocf)]]` for C++.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D132302