[llvm] Fix MSVC warning in benchmark (PR #147357)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

The Clang MSVC build seems to be green: https://lab.llvm.org/buildbot/#/builders/63
But the Flang one is red because of these changes, which is... interesting. Oh... this enumeration comes from `winnt.h` so I wonder if there's a difference in SDKs??

https://github.com/llvm/llvm-project/pull/147357