Suppressing the C4324 warnings generated by MSVC. This is the only declarative...

Suppressing the C4324 warnings generated by MSVC. This is the only declarative instance that would generate the warning, but it accounted for about 525+ warnings due to template instantiations. This is a marginal-value warning which we may decide to disable more broadly, but since this header is in Support and may be used out of tree, it's a low burden for us to be warning-free in this case.

llvm-svn: 235219