WinSDK: extract System submodule

[egorzhdan]

Currently winioctl.h header gets included into WinSDK.WinSock2 via windows.h & winscard.h.
MSDN lists this header in https://docs.microsoft.com/en-us/windows/win32/api/_base/ along with DbgHelp.h, so this change also moves WinSDK.DbgHelp to the System submodule.

[compnerd]

Mind naming this IOCTL to match https://docs.microsoft.com/en-us/windows/win32/api/winioctl/ ?

[lxbndr]

Is there some general rule for naming these modules? I imagine me writing something in Swift and thinking "Ok, MSDN says that these API are defined in winioctl.h, but what import do I need to add?"
At this point it looks easier to just look into modulemap and search for corresponding module.
I am torn between Swiftified module names and "platformish" MSDN-inspired names.

[egorzhdan]

@lxbndr
From the compilation POV it doesn't really matter which import you add, since all of the submodules are represented by a single underlying Clang module, so importing any submodule (or the entire WinSDK) implicitly imports them all.

From the convenience POV when writing the imports, as you've pointed out, looking for a specific submodule names might be frustrating, if you prefer them over importing the entire module. Ideally your IDE or editor would automatically insert the desired import depending on your preference though, and you wouldn't have to remember the submodule names.
When reading the imports, especially by someone who isn't an expert in the Windows API naming & structure, Swifty names IMO would be easier to understand than some cryptic Windows abbreviations.

There was also a related discussion in #33929, where @compnerd was in favor of the MSDN section names, which don't usually match the header names, and are closer to the Swifty names.

[lxbndr]

@egorzhdan Oh, thanks for pointing on that discussion. Compnerd's suggestion has sense.

[compnerd]

@swift-ci please smoke test

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test Windows platform

[compnerd]

@swift-ci please test Windows Platform