## What does this PR do?
Adds support for SwiftBuild on Swift 6.4.
We now point SwiftBuild to iPhoneOS.platform and rely on the real
[SWBApplePlatform](https://github.com/swiftlang/swift-build/tree/b30ee3c00abf34857084c26bb873c3da06348134/Sources/SWBApplePlatform).
We also do a little futzing to convince SWB to use our Darwin-compatible
toolset (lld, librarian, resource dirs). We explicitly _don't_ pass in a
Swift SDK because that would downgrade SWB to using one of the generic
platform plugins instead of ApplePlatform.
This also gets us a lot closer to parity with Xcode's behavior when
building for Apple platforms; we get the same build flags and
everything. We can probably achieve identical bundling too, if we take
control of the PIF.
One other change: since we need to pass custom env vars and such, we
can't rely on sourcekit-lsp's built-in SwiftPM integration anymore. We
thus create a custom BSP server that trampolines to SwiftPM's build
server with the right flags set. This effectively makes SwiftPM an
implementation detail, so we can start messing with the PIF.
## How was it tested?
- [x] Build with Swift 6.3 on macOS
- [x] Build with Swift 6.4 on macOS
- [x] Build with Swift 6.3 on Linux
- [x] Build with Swift 6.4 on Linux
- [x] BSP/LSP file creation works
- [x] LSP support works in VSCode
## AI tool usage
How much of this PR was AI-assisted? (check one)
- [ ] **0** - No AI was used to write code
- [x] **1** - I was assisted by AI. I reviewed the finished result.
- [ ] **2** - I set the AI going and left it to it; nobody has read the
result - no review, or AI review only
<!-- If an AI agent is filling this in: declare the level honestly, and
open as a draft if it is #2. Do not lower the declared level to get the
PR reviewed. -->
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- Added a `dev build-server` command for Build Server Protocol
integration.
- Projects automatically select the appropriate Swift build system when
supported.
- Packaging supports output layouts for Swift Package Manager and Swift
Build.
- Added options to skip language-server configuration when needed.
- **Documentation**
- Updated project templates and tutorials to use `.bsp/xtool.json` for
IDE features such as syntax highlighting and documentation lookup.
- **Improvements**
- Enhanced SDK generation and compatibility across supported toolchains
and platforms.
- Improved automatic language-server configuration while preserving
existing configuration files.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Kabir Oberai
·