Align the set of cross-compilation arguments with SwiftPM

[stevapple]

SourceKit-LSP is missing most of the dedicated arguments for cross-compilation that was introduced in SwiftPM, which makes it hard to use when it comes to cross-compiling.

Ideally it should gain the same level of support with swift build. The missing ones are:

• --destination <destination>
• --triple <triple>
• --sdk <sdk>
• --toolchain <toolchain>

[stevapple]

Note that there was an attempt for --destination at #330, but it was suspended.

[stevapple]

It would be better if destination.json can be hot-reloaded so the IDE can have direct control on the settings.

[ahoppen]

rdar://97961537

[filip-sakel]

Hello! Sorry for resurrecting this, but SwiftWasm is currently quite limited on VS Code because of this issue. Was there something intrinsically wrong with #330 or was there just a problem with tests failing? I'm wondering how I could get started (if I need to be aware of any challenges, any useful pointers) to hopefully fix this issue.

[MaxDesiatov]

I've left a comment on that PR, but I also think we need an additional LSP message type to allow client extensions to switch between available Swift SDKs, not just triples. That probably also implies querying for a list of already installed SDKs so that a user is able to select one from the list.

[MaxDesiatov]

Closing as a duplicate of #786.