Only emit ABI descriptors on Darwin platforms

Where Library Evolution is enabled

Author: artemcm

Sources/SwiftDriver/Driver/Driver.swift

@@ -360,6 +360,11 @@ public struct Driver {
     guard isFeatureSupported(.emit_abi_descriptor) else {
       return nil
     }
+    // Emit the descriptor only on platforms where Library Evolution is supported,
+    // or opted-into explicitly.
+    guard targetTriple.isDarwin || parsedOptions.hasArgument(.enableLibraryEvolution) else {
+      return nil
+    }
     guard let moduleOutput = moduleOutputInfo.output else {
       return nil
     }

Tests/SwiftDriverTests/APIDigesterTests.swift

@@ -229,6 +229,9 @@ class APIDigesterTests: XCTestCase {
   }
 
   func testBaselineComparisonJobFlags() throws {
+#if !os(macOS)
+    throw XCTSkip("Skipping: ABI descriptor is only emitted on Darwin platforms.")
+#endif
     do {
       var driver = try Driver(args: ["swiftc", "foo.swift", "-emit-module", "-compare-to-baseline-path", "/baseline/path",
                                      "-sdk", "/path/to/sdk", "-I", "/some/path", "-F", "framework/path",