@cdecl enums are Swift enums representable in C. These enums must have
an integer raw type. They can be referenced from @cdecl functions and
@objc methods. @objc enums are still rejected from @cdecl functions.
This implements basic checks on the validity of the @cdecl attribute and
ensures the parameters and result types are representable in C. Many
more diagnostics will need to be updated to verify full representability
in C.
`Int` on Windows x86_64 is different from `Int` on other targets due to
it being a LLP64 target. Explicitly use the `Int32` type for the
enumeration to fix the test.
There's an immediate need for this in the core libs, and we have most of the necessary pieces on hand to make it easy to implement. This is an unpolished initial implementation, with the following limitations, among others:
- It doesn't support bridging error conventions,
- It relies on ObjC interop,
- It doesn't check for symbol name collisions,
- It has an underscored name with required symbol name `@cdecl("symbol_name")`, awaiting official bikeshed painting.
