All symbols are private (as in the API descriptor) if the library level
is more restricted than `LibraryLevel::API`.
Resolves rdar://164901718
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- Add SPI availability information to `APIAvailability` from attribute
`@_spi_available`.
- Correctly obtain effective availability for nested declarations
without immediate availability attributes.
Resolves rdar://159702280
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api generation
This resolves the bug where apijson files recorded declarations as
unavailable purely because it was unavailable on a unrelated platforms.
Resolves: rdar://113552185
