ModuleInterface: Avoid printing @_spi enum elements in public swiftinterfaces

[tshortli]

The @_spi attribute was accepted on enum case declarations but the cases were still printed in public swiftinterfaces, leaking the SPI. The compiler now avoids printing enum cases in public swiftinterfaces when they are declared as @_spi by checking whether the enum case declaration contains any enum elements that are SPI (enum case declarations apparently do not themselves have a copy of the attributes they were written with). The compiler now also diagnoses @_spi on enum case declarations when the enum is @frozen since all the cases of a frozen enum must be known to the client so that the memory layout for the enum can be computed.

This PR also includes tests that show that the exportability of case statements is not checked. 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. A follow up PR will be needed to address this.

Resolves rdar://72873771

[tshortli]

This file was deleted because the tests it contains seem to be completely superseded by the spi_members.swift test.

[tshortli]

I removed this because it interfered with the stability of printing the interface with an enum case declaration like case a, b. It seemed safe to remove to me because there are many other tests exercising serialization and deserialization of the @_spi attribute simply by compiling a helper module with @_spi declarations. If this is still needed for some reason, though, I can split it into a new test case.

[tshortli]

@swift-ci please test