[cmake][msvc] Enable standards-conforming preprocessor

Since we now only support Visual Studio 2019 16.7 and newer, we're able to use the /Zc:preprocessor flag that turns on the standards-conforming preprocessor. Among other things, it correctly expands __VA_ARGS__. This enables removal of some boilerplate in D135128.

Reviewed By: Bigcheese, thieta

Differential Revision: https://reviews.llvm.org/D135128