```
@_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
