- 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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The relationship between the code in these two libraries was fundamentally circular, indicating that they should not have been split. With other changes that I'm making to remove circular dependencies from the CMake build graph I eventually uncovered that these two libraries were required to link each other circularly, but that had been hidden by other cycles in the build graph previously.
