[Sema] Report conflicting uses of @_spiOnly with other import modifiers

[xymus]

Check that an @_spiOnly attribute isn't used with @_implementationOnly or @_exported on the same import statement or in the same source file. This check is partially integrated with the existing check for @_implementationOnly consistency. This could be integrated further once we have a per-file implementation-only import alternative, for now the differences between @_implementationOnly and @_spiOnly makes it a bit messy.

For clarification, here's what we check:

• (updated) Raise an error if incompatible import modifiers are used on the same import statement. Only one or none of the following can be used at a time: @_spiOnly, @_implementationOnly and @_exported.
• (new) Raise an error on inconsistent @_spiOnly imports of the same target within one source file.
• (old, for reference) Report as a warning inconsistent @_implementationOnly imports of the same target within a module.

Follows the integration with API checks from #61023. This should complete the main feature, but of course, further tweaks will likely be needed.

[xymus]

@swift-ci Please smoke test

[tshortli]

Any reason this is based off of a hardcoded string instead of being a %select{...} in the diagnostic? I don't know if this is an eventual goal, but it seems like it would be easier to localize a diagnostic that uses select.

[xymus]

It was an oversight. The last commit should update it to the usual pattern. The ImportFlags enum is sparse as it's a kind of option set, so it required more logic than usual.

[xymus]

@swift-ci Please smoke test