Refine access note descriptions in diagnostics

[beccadax]

This PR will finalize the way diagnostics describe when they're caused by access notes.

Specifically, the pre-existing access note remarks are now only emitted for valid attributes; for invalid attributes, the diagnostic is augmented with text explaining that it was added for an access note and has been disabled:

class MyClass: NSObject {
    // Both of these methods have ObjC: true in access notes

    func good(_: Int) {}
    // remark: implicitly added '@objc' to this method, as specified by access note for <reason string>
    // note: add '@objc' explicitly to silence this warning

    func bad(_: Int?) {}
    // remark: ignored access note: the type of the parameter cannot be represented in Objective-C;
    //         did not implicitly add '@objc' to this method, even though it was specified by access
    //         note for <reason string>
}

To accomplish this behavior, this PR makes it possible for a diagnostic to take a DiagnosticInfo * as a parameter; it then adds an InFlightDiagnostic::wrapIn() method which uses this capability. Thus, an error like diag::objc_invalid_on_func_single_param_type can be wrapped in diag::wrap_invalid_attr_added_by_access_note to add verbiage relating to access notes.

This PR also:

• Emits one access note success remark covering all attributes added to each declaration, instead of separate ones for each attribute.
• Softens selector conflicts caused by access notes into remarks.
• Adds a command-line option which controls whether and how access note remarks are emitted; in particular, you can use it to strengthen ignored access notes back to errors.

I will probably be tweaking this further before I'm finished with this PR.

Fixes rdar://71872575.

[beccadax]

@swift-ci please test

[swift-ci]

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[beccadax]

@swift-ci please test

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Git Sha - ccf86c46b34d9c25d8d02d93f40200c157989a76

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[xymus]

Can you tell me more about the advantage of wrapping this diagnostic in the Xcode display? Should we apply the same trick to more kind of diagnostics or is this specific to access notes?

[beccadax]

Basically, Xcode's inline error UI doesn't display notes unless they have fix-its. Wrapping a diagnostic produces a longer message, but it's a single message that will actually be made visible to users in the UI they most often use to read diagnostics.

I've been thinking that it might be good to turn diag::module_interface_build_failed and friends into wrapping diagnostics around the underlying errors, since the underlying errors often seem to go unnoticed by users, but are usually a better way to resolve the problem. But that's a different pull request.

[xymus]

This looks good! I'll be curious to see a demo of it with all of the new diagnostics refinements.

[beccadax]

@swift-ci please test

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Build failed
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Git Sha - 5cd34ee37e0250d3fad297ed7a92e5652009bffe

[beccadax]

@swift-ci smoke test and merge

[beccadax]

@swift-ci smoke test and merge