Setting to specify the Visual Studio tool directory

[DougGregor]

Is your feature request related to a problem? Please describe.
If there are multiple versions of Visual Studio installed, Swift (via Clang) will pick one of them based on a complicated series of checks for environment variables / path / config files / registry. If it picks the wrong one, it's hard to know how to select the one you wanted.

Describe the solution you'd like
Can we add a setting to the extension that lets us directly specify the path to the Visual Studio tools that Swift should use? The most straightforward way seems to be to set the VCToolsInstallDir environment variable.

Describe alternatives you've considered
Setting the environment outside of VS Code, in tasks.json, or with unsafe flags in SwiftPM. None of them are as discoverable as a setting would be, especially given that we have an SDK path setting already.

[plemarquand]

Would using the swift.swiftEnvironmentVariables setting to set VCToolsInstallDir be enough here? If so, we can more formally document this as an approach to select which version of Visual Studio is used.

[DougGregor]

That's probably fine if we document it somewhere that folks can find.

[plemarquand]

This is now documented in the Extension Settings documentation.