SwiftDriver: add support for baremetal targets (to reflect swiftlang/swift#35970)

Author: kubamracek

Sources/SwiftDriver/Driver/Driver.swift

@@ -2892,12 +2892,16 @@ extension Driver {
 
 extension Triple {
   func toolchainType(_ diagnosticsEngine: DiagnosticsEngine) throws -> Toolchain.Type {
+    guard let os else {
+      diagnosticsEngine.emit(.error_unknown_target(triple))
+      throw Driver.ErrorDiagnostics.emitted
+    }
     switch os {
     case .darwin, .macosx, .ios, .tvos, .watchos:
       return DarwinToolchain.self
     case .linux:
       return GenericUnixToolchain.self
-    case .freeBSD, .haiku, .openbsd:
+    case .freeBSD, .haiku, .openbsd, .none:
       return GenericUnixToolchain.self
     case .wasi:
       return WebAssemblyToolchain.self

Sources/SwiftDriver/Utilities/Triple+Platforms.swift

@@ -282,9 +282,10 @@ extension Triple {
   ///   identified as just `darwin` instead of by individual platform names.
   ///   Defaults to `false`.
   public func platformName(conflatingDarwin: Bool = false) -> String? {
-    switch os {
-    case nil:
+    guard let os else {
       fatalError("unknown OS")
+    }
+    switch os {
     case .darwin, .macosx, .ios, .tvos, .watchos:
       guard let darwinPlatform = darwinPlatform else {
         fatalError("unsupported darwin platform kind?")
@@ -314,6 +315,8 @@ extension Triple {
       return "haiku"
     case .wasi:
       return "wasi"
+    case .none:
+      return nil
 
     // Explicitly spell out the remaining cases to force a compile error when
     // Triple updates

Sources/SwiftDriver/Utilities/Triple.swift

@@ -1109,6 +1109,7 @@ extension Triple {
     case hurd
     case wasi
     case emscripten
+    case none
 
     var name: String {
       return rawValue
@@ -1188,6 +1189,8 @@ extension Triple {
         return .wasi
       case _ where os.hasPrefix("emscripten"):
         return .emscripten
+      case _ where os.hasPrefix("none"):
+        return Triple.OS.none
       default:
         return nil
       }