Windows installer should check for Visual Studio and prompt the user to install it (at least)

[al45tair]

Motivation

Without reading the additional Instructions page, or thinking it doesn't apply to them because they are using a newer version of Swift, users may try to install just Swift (and nothing else) on a machine using the GUI installer. Unless they also install Visual Studio and the Desktop C++ development components, this will not work — Swift requires the C runtime library and headers, as well as a Windows SDK and those are installed by Visual Studio.

Proposed solution

The GUI installer should check whether Visual Studio components are installed already, and if not, prompt the user to install them.

Alternatives considered

The GUI installer could try to install the Visual Studio components itself, though this might be complicated to achieve in practice (we may need to detect available versions of things, and there may be licensing issues too).

Additional information

I filed swiftlang/swift-org-website#763 to improve the Windows installation page as well.