Add diagnostic group names to several import-related warnings

[tshortli]

• Explanation: Adds group names to several import-related warnings so that the diagnostics can be controlled via -Werror and -Wwarning.
• Scope: Affects projects in which the modified warnings are emitted (though adding a warning group name to a diagnostic that previously did not have one should generally have no effect on existing projects).
• Issues: rdar://147656067&175381846
• Risk: Low since adding warning group names to diagnostics that previously did not have them should generally have no effect on existing projects
• Testing: Modified existing test cases to verify that the behavior of the diagnostics can now be controlled.

[tshortli]

@swift-ci please smoke test

[finagolfin]

There was a recent break in Testing, this pull appears to fix it:

swiftlang/swift-testing#1688

@swift-ci smoke test macOS

[tshortli]

There was a recent break in Testing, this pull appears to fix it:

swiftlang/swift-testing#1688

@swift-ci smoke test macOS

I don't think that's right. This just adds some diagnostic groups to existing diagnostics.

[finagolfin]

Right, but that Testing issue had broken all macOS PR CI earlier, completely unrelated to this diagnostic pull.

[tshortli]

Ok, I see now, I misinterpreted which pull request you were referring to as fixing the issue. It's the swift-testing one that fixed it.