[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?