[SwiftWarningControl] Add support for file-scoped 'using @warn'

[artemcm]

using @warn(DiagGroupID, as: warning)
specifies a file-scoped diagnostic group control. This change adds detection of these controls and upon encountering one, adds a corresponding control to the file-scoped root of the warning control region tree.

This change also removes the optimization on warningGroupControlRegionTreeImpl construction which only constructs a tree that considers nodes which contain a queried location because we now have a scenario where diagnostic group behavior at a specific location can be affected by an using @warn node elsewhere in the file. I expect clients to run queries against a once-built tree for the source file, so I do not think this will have a meaningful impact, in practice.

[ahoppen]

One high-level comment before I dive into a detailed review: Is using @warn only allowed at the top level or also inside declarations?

• If it is only allowed at the top level: We could still have an API that takes a position and just doesn’t walk into declarations
• If it is allowed inside type declarations: Does the using declaration only apply within that type, similar to applying it as an attribute to the type or does it still apply to the whole file?

[artemcm]

• If it is only allowed at the top level: We could still have an API that takes a position and just doesn’t walk into declarations

This is correct, using @warn is only allowed at the top level. I've restored the position API and changed the logic to specifically skip Declarations which do not contain the specified position and it seems to work well. Thanks.

[artemcm]

@swift-ci test

[ahoppen]

Could you add a test case that contains using @warn inside a declaration to make sure that it is ignored / diagnosed / handled in a way that makes sense to you?

[ahoppen]

Suggested change

	let experimentalFeatures = experimentalFeatures ?? Parser.ExperimentalFeatures(rawValue: 0)
	let experimentalFeatures = experimentalFeatures ?? []