These tests identified a gap in type checking the exportability of case statements. Unfortunately this gap is not simple to close, since exportability is usually checked during availability checking but availability checking isn't appropriate for case statements and there isn't existing infrastructure to check exportability independently.
```
@_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
