Add swift toolchain header file on CI to build runtime part on CI

[Kyle-Ye]

Possible 1:

1. Clone the swiftlang/swift and swiftlang/llvm repo.
2. Specify a configuration to make a header-only build. (Only produce the def and header file, skip the actual .o build)
3. Update some scripts to make the CI build's OPENGRAPH_SWIFT_TOOLCHAIN_PATH variable points to the toolchain dir.

Possible 2:

1. Generate the necessary header for macOS, iOS, Linux locally and then store the headers in another git repo. (Update it when new Swift version is released)
2. Clone the toolchain header repo
3. Update some scripts to make the CI build's OPENGRAPH_SWIFT_TOOLCHAIN_PATH variable points to the toolchain dir.

[Kyle-Ye]

I'd love to see the SwiftUI diffing and rendering tools on other platforms, is there any way I can help?

Hi @Lancelotbronner, this is an easy and help wanted one for contributing to OpenGraph.

If you want try it or need some help, feel free to comment below.

[Lancelotbronner]

I don't know much about CIs but I can start learning!