Un-revert #51 and fix Windows build errors

[QuietMisdreavus]

Reverts #53

This brings back the upstream changes, effectively re-landing #51. I'm keeping this open while i look at the Windows CI failures that happened for the Swift compiler.

[QuietMisdreavus]

cc @compnerd; MSVC seems to be more stringent about symbol visibility mismatches than Clang. I couldn't even find an equivalent warning to enable in Clang to test this on macOS 😅

[compnerd]

Yeah, MSVC is far better about bonking you over the head when you get things wrong. -Werror=inconsistent-dllimport-dllexport I think is the closest and its not nearly as good.

[QuietMisdreavus]

The dllimport/dllexport attributes are only added on Windows, though, so there would need to be something for the equivalent __attribute__((__visibility__("default"))) (or "hidden") added on Mac/Linux. 🙃 Unfortunately, since the "exported" flag uses the default visibility, there's nothing to warn about when the attribute is missing. I tried adding -fvisibility=internal or something like that, but it didn't seem to affect anything.

[QuietMisdreavus]

It looks like the cross-repo testing for Windows in swiftlang/swift#65518 worked, so this should be able to land without breaking the Swift compiler's build any more. 🤞

[d-ronnqvist]

These changes look good to me but I haven't tested them in a Windows build.