Align Clang resource directory lookup with Clang driver logic

Sources/SwiftDriver/Jobs/Toolchain+LinkerSupport.swift

@@ -26,12 +26,7 @@ extension Toolchain {
     for targetInfo: FrontendTargetInfo,
     parsedOptions: inout ParsedOptions
   ) throws -> VirtualPath {
-    var platform = targetInfo.target.triple.platformName(conflatingDarwin: true)!
-    // compiler-rt moved these Android sanitizers into `lib/linux/` a couple
-    // years ago, llvm/llvm-project@a68ccba, so look for them there instead.
-    if platform == "android" {
-      platform = "linux"
-    }
+    let platform = targetInfo.target.triple.clangOSLibName
 
     // NOTE(compnerd) Windows uses the per-target runtime directory for the
     // Windows runtimes. This should also be done for the other platforms, but

Sources/SwiftDriver/Utilities/Triple+Platforms.swift

@@ -303,7 +303,28 @@ extension Triple {
     }
   }
 
-  /// The platform name, i.e. the name clang uses to identify this target in its
+
+  /// The "os" component of the Clang compiler resource library directory (`<ResourceDir>/lib/<OSName>`).
+  /// Must be kept in sync with Clang driver:
+  /// https://github.com/llvm/llvm-project/blob/llvmorg-20.1.4/clang/lib/Driver/ToolChain.cpp#L690
+  @_spi(Testing) public var clangOSLibName: String {
+    guard let os else {
+      return osName
+    }
+    if os.isDarwin {
+      return "darwin"
+    }
+
+    switch os {
+    case .freeBSD: return "freebsd"
+    case .netbsd: return "netbsd"
+    case .openbsd: return "openbsd"
+    case .aix: return "aix"
+    default: return osName
+    }
+  }
+
+  /// The platform name, i.e. the name Swift uses to identify this target in its
   /// resource directory.
   ///
   /// - Parameter conflatingDarwin: If true, all Darwin platforms will be

Tests/SwiftDriverTests/TripleTests.swift

@@ -1317,6 +1317,15 @@ final class TripleTests: XCTestCase {
                                 shouldHaveJetPacks: true)
   }
 
+  func testClangOSLibName() {
+    XCTAssertEqual("darwin", Triple("x86_64-apple-macosx").clangOSLibName)
+    XCTAssertEqual("darwin", Triple("arm64-apple-ios13.0").clangOSLibName)
+    XCTAssertEqual("linux", Triple("aarch64-unknown-linux-android24").clangOSLibName)
+    XCTAssertEqual("wasi", Triple("wasm32-unknown-wasi").clangOSLibName)
+    XCTAssertEqual("wasip1", Triple("wasm32-unknown-wasip1-threads").clangOSLibName)
+    XCTAssertEqual("none", Triple("arm64-unknown-none").clangOSLibName)
+  }
+
   func testToolchainSelection() {
     let diagnostics = DiagnosticsEngine()
     struct None { }