This is the clang part of the initial implementation to support Windows ARM64 COFF format.
Here is the llvm part of this patch: https://reviews.llvm.org/D34705
I cannot imagine there will be MicrosoftARM64beTargetInfo, so I wonder if we should name this MicrosoftARM64TargetInfo.
LGTM. I don't think I have enough knowledge to sign off, but as Saleem has already LGTM'ed, I guess it should be okay.