Suppress warning about missing SDKSettings.json for non-Darwin platforms

[kateinoigakukun]

The SDKSettings.json file is only used for Darwin platforms, so we should not emit a warning about it being missing for non-Darwin platforms.

[kateinoigakukun]

@swift-ci test

[al45tair]

Interesting. The other option would be to put an SDKSettings.json file in your SDK. I believe the minimal set of keys is

{
  "DisplayName": "<display name here>",
  "Version": "<SDK version>",
  "VersionMap": {},
  "CanonicalName": "<canonical name here>"
}

Everything else is optional. I don't have a strong opinion either way; @MaxDesiatov, WDYT? At least in principle, it would be nice to have Xcode support for the SDKs at some point, and including an SDKSettings.json file might make sense for that?

[MaxDesiatov]

It feels weird to me to require all Swift SDKs to include Darwin-specific stuff in them. I'd rather see the file itself marked as optional and not trigger any warnings anymore.

[kateinoigakukun]

Placing SDKSettings.json in SDKs is definitely an option, but we can't always modify SDK contents (e.g. Android NDK is not under our control. / root directory can be used as SDK on non-darwin systems)
So I think making the file itself optional seems reasonable on non-Darwin platforms. For future IDE support, we can provide such metadata from Swift SDK's metadata file.