Turns out we need this directory to pass to the -frontend command
for creating the interop header, so refactor the whole find module
to find it on each platform.
Add the proper annotations for the Cell and Cell::Visitor classes to be
visible in Swift. This lets us remove some OpaquePointer shinangians in
the Swift bindings.
This confirmed works on Xcode 16, and Xcode 16.1 Beta 2, with CMake 3.28
or higher.
On linux, the 6.0.0 release from swiftly is still missing my libstdc++
workaround, so it needs a snapshot to work.
This project is a part of the Xcode-shipped toolchain on macOS, but
needs built from source on other platforms. However, using the Xcode
version of the framework leads to a bunch of rpath confusion when
trying to link it the expected way. I suspect that there will be a
more intuitive way to link this library from the toolchain when it
stabilizes. So we'll build it everywhere :)
