Add --destination option for cross-compiled projects

[MaxDesiatov]

Currently SourceKit-LSP can't be used for projects that can't be compiled on a host platform and need to be cross-compiled with an appropriate destination file passed to swift build. This PR adds a --destination option to SourceKit-LSP that is then passed to SwiftPM workspaces, which in my undestanding is equivalent to passing --destination to the underlying swift build invocations.

[MaxDesiatov]

@swift-ci please test

[MaxDesiatov]

@swift-ci please test Linux platform

[benlangmuir]

We should add a test to SwiftPMWorkspaceTests for this. I think you could take a test like testBuildSetup and instead of checking the customization of BuildFlags, check the customization on the destination.

[MaxDesiatov]

It does not seem to work as I expected, Looks like initializing a Workspace with these arguments is not fully equivalent to the swift build --destination CLI invocation, the inferred SDK path is not the same, which leads to issues. I'm closing this for now until a better solution is found.