Sema: Narrow fix to allow `@_spi_available` in extensions
Allow referencing an `@_spi_available` decl in extensions to
`@_spi_available` types. This is a narrow fix as it should really be
handled as part of the context check but that check is currently too
permissive.
Fow now let's narrowly allow legal code. And then we should look at
revisiting the SPI availability logic, separate it from normal SPI and
treat more like availability.
Adding a test comparing the behavior of `@_spi` with `@_spi_available`
to document the current implementation.
rdar://159292698
