Its functionality has been superseded by `@_spiOnly import`. There are no
longer any known clients and the flag was already unsupported in Swift 6, so
the functionality is now removed (but the flag is only deprecated for Swift 5).
Resolves rdar://136867210.
Enabling `@_spiOnly` also enables stronger type-checking of SPI decls.
As this could be source breaking, it has always been opt-in. Turn it on
by default in Swift 6 mode where the stronger type-checking will also
become expected.
At the same time, disable the old alternative to `@_spiOnly` which was
designed to be compatible with old compilers. Any user of that feature
should move to `@_spiOnly` or `package import` instead.
The patch introduces a new setting instead of changing existing settings
because the generated interfaces in the IDE have slightly different
requirements; the extended type there is unconditionally not printed
qualified (even if it is ambiguous). This is likely because the
ambiguity heuristic is very weak; it doesn't even do name lookup.
Simplifying that logic would be nice, but then we'd need to update
a bunch of IDE/print* tests and end up with more more visual clutter
in the IDE.
Introducing the new setting means we can change the behavior for
swiftinterface files without affecting the behavior for IDE interfaces.
Fixes rdar://79093752.
Print implementation-only imports in the private textual interface
only if also importing SPI. This allows to export types from
implementation-only imports in SPI and brings the private textual
interfaces in line with the binary interfaces.
This is a temporary solution as we need to better design the language
feature around this.
This feature requires passing -experimental-spi-imports to the frontend
that generates the private swiftinterface file.
