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.
```
@_specialize(exported: true, spi: SPIGroupName, where T == Int)
public func myFunc() { }
```
The specialized entry point is only visible for modules that import
using `_spi(SPIGroupName) import ModuleDefiningMyFunc `.
rdar://64993425
Extensions to implementation-only types are accepted at type-checking
only if they don't define any public members. However if they declared a
conformance to a public type they were also printed in the
swiftinterface, making it unparsable because of an unknown type.
Still accept such extensions but don't print them.
rdar://problem/67516588
When emitting the private swiftinterface, the compiler prints the
attribute explicitly even when it is deduced from the context. This can
lead to unparsable private swiftinterface files.
As a narrow fix, check if the decl type is supported before printing the
attribute.
rdar://64039069
Using a SetVector fixes an issue where many source files imported the
same SPI group from the same module, the emitted private textual
interfaces superfluously repeated the `@_spi` attribute on the import.
rdar://problem/63681845
