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
