Support the new MSVC preprocessor

[JosJuice]

Intends to fix https://bugs.dolphin-emu.org/issues/12170.

Untested, because I'm using VS 16.5 to avoid the std::result_of problem, and as far as I know the new preprocessor requires VS 16.6.

[BhaaLseN]

FYI, I tried building this branch on 16.6.2 (along with /std:cpp17 and /experimental:newLambdaProcessor because it wouldn't build at all otherwise), and it works without /Zc:preprocessor. However, once I add that, it fails inside ucrt_math.h (included by pch) with Windows SDK 10.0.17763.0. filoppi confirmed on IRC that it does work with Windows SDK 10.0.19041.0 though, so we might also have to consider documenting this somewhere. We used to have the version in README.md, but since switched it to just say "Windows 10 SDK".

[JosJuice]

filoppi confirmed on IRC that it does work with Windows SDK 10.0.19041.0 though, so we might also have to consider documenting this somewhere.

Since we're not actually enabling /Zc:preprocessor, is there really a need to document it?

[BhaaLseN]

I'm guessing it would become the default at some point, for compatibility with other compilers? I mean, good enough for me if we want to handle it if (or when) it comes up.

[AdmiralCurtiss]

Is there anything stopping us from upgrading Qt, by the way?

[JosJuice]

No, other than people not knowing how to or wanting to build the new version to put in Externals.