[clang][cli] Simplify repetitive macro invocations

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. It (among other things) correctly expands `__VA_ARGS__` (see https://learn.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview?view=msvc-170#macro-arguments-are-unpacked). This enables us to get rid of some repetitive boilerplate in Clang's command-line parser/generator.

Reviewed By: Bigcheese

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