Simplify capture representations

[milseman]

Make CaptureStructure semi-vestigial and introduce an array-of-structs CaptureList to use instead. Flipping from singly to multiply-nested captures is just flipping a Bool, though we still need to feed that info into the existentially stored captures.

[milseman]

@hamishknight I see

@available(*, deprecated, message: "moving to SwiftCompilerModules")
func libswiftParseRegexLiteral(

What is the compiler-library interaction story? Is this function called or does the logic have to live outside the library?

[hamishknight]

I believe those libswift... functions can be removed now that they've been moved into the compiler repo (cc @rintaro)

[milseman]

But what does that mean WRT the interface? We really wanted to keep this logic in the library because it's super easy to break otherwise. Case in point, I think this PR would break it because it makes CaptureStructure internal

[milseman]

At the very least we need a comment convention for what is a compiler-locked API and what isn't. We could do that via _spi actually, which would be nice.

[hamishknight]

Yeah I think it makes sense to add back complier entry points, which should be the only thing the compiler calls into (and yeah we could make them SPI). We can keep the bridging gunk on the compiler side tho.

[milseman]

Do you think you can commit those changes to this PR? I'm not the most up to date.

[milseman]

@swift-ci please test