[APIGen] Handle SPI availability for nested decls

[zixu-w]

• Add SPI availability information to APIAvailability from attribute @_spi_available.
• Correctly obtain effective availability for nested declarations without immediate availability attributes.

Resolves rdar://159702280

[tshortli]

This may be fine for now, but FWIW getInnermostDeclWithAvailability() is probably not going to give you correct information for 100% of cases. For example:

@available(macOS 26, *)
public struct Outer {
  @available(iOS 26, *)
  public struct Inner {
    public func f()
  }
}

f() is implicitly introduced in macOS 26 but getInnermostDeclWithAvailability() is going to find Inner which only has an attribute for iOS. Correctly finding the attribute that is responsible for the platform availability of a decl on the current platform is a pretty tricky task to get right.

[zixu-w]

Oh that's a really good point. Forgot that we are also matching platforms here. But the current code doesn't correctly handle this case as well.
I think we'll have to traverse up until we hit an availability attribute with the matching platform, or the top level context right?

[tshortli]

Yeah, to be clear this seems like an improvement over the existing approach and would be fine to land as-is if you want to make incremental progress. To really make it as accurate as possible, you'll probably need to re-implement something like getSemanticAvailableRangeDeclAndAttr() here. The other (existing) problem with this code is that it doesn't handle the case where availability is inherited from an attribute for a different platform:

// Implicitly @available(visionOS 26, *) too
@available(iOS 26, *)
public struct Outer { }

[cyndyishida]

I'm also fine with this as is.

but please file a bug report so we don't forget about known bugs mentioned in

problem with this code is that it doesn't handle the case where availability is inherited from an attribute for a different platform:

// Implicitly @available(visionOS 26, *) too
@available(iOS 26, *)
public struct Outer { } 

and

probably not going to give you correct information for 100% of cases. For example:

@available(macOS 26, *)
public struct Outer {
  @available(iOS 26, *)
  public struct Inner {
    public func f()
  }
}

f() is implicitly introduced in macOS 26 but getInnermostDeclWithAvailability() is going to find Inner which only has an attribute for iOS.

[zixu-w]

@swift-ci please test