build: use C++14 for Windows builds

[compnerd]

The Windows SDK headers require C++14 or newer features. Visual Studio
is lax in enabling these features unlike clang. Explicitly upgrade to
the higher standard for the Windows cross-compilation.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @troughton

[compnerd]

@swift-ci please test

[troughton]

@compnerd I already have this in #13140, although mine’s a slightly different approach. Could you take a look at that before you merge this? This looks a little simpler so I could modify my PR to match (or remove those changes from mine).

[compnerd]

@troughton #13140 changes the shared build which is used for the standalone builds. Did you actually test that portion of it? Also, I think that splitting up that PR into individual pieces is better as not all of it can be merged as is. This is ready to go ... but, I suppose that I can wait if you really want to split it up and submit a separate PR.

[jckarter]

Concerns about MSVC support have long been the reason we've held out ratcheting up to C++14. If Microsoft's SDK requires C++14 now, is there any reason for us not to just move to C++14 unilaterally for all targets? I'd love to be able to use digit separators in my hex literals…

[compnerd]

@jckarter fine by me, but that probably warrants an email to the ML to ensure that everyone is okay with that (I guess @tkremenek probably would make the call?).

[troughton]

I can say that right now there’s something in the Swift project that fails on Linux with a C++14 standard, although I can’t remember what that was and won’t have the means to test that for a few hours.
I’m not sure whether it’d be a simple thing to fix or not.

As for this PR specifically: @compnerd I tested my changes only through the build-script invocation - I assume that by a standalone build you mean with a CMake invocation directly? And I don’t have any objection to you merging this in if you think it’s a better approach.

[compnerd]

@troughton yeah, there is a mode to build just the SDK overlay with cmake. I think that leaving that untouched for now is a better option. I think that unblocking the Windows build is probably a good thing too. We can remove this when we move the entire project over to C++14, which might take a little bit of work as you point out that there are some issues. I'll merge this for now as a result, and work on moving the entire project to C++14 separately.