Marks a declaration as SPI (System Programming Interface), instead of API. Modules exposing SPI and using library evolution generate an additional .private.swiftinterface file (with -emit-private-module-interface-path) in addition to the usual .swiftinterface file. This private interface exposes both API and SPI. Clients can access SPI by marking the import as @_spi(spiName) import Module. This design makes it easy to find out which clients are using certain SPIs by doing a textual search.