Accept none as the value for -runtime-compatibility-version

rdar://71942877

Author: nkcsgexi

Sources/SwiftDriver/Driver/Driver.swift

@@ -2082,7 +2082,8 @@ extension Driver {
         if let version = SwiftVersion(string: versionString) {
           info.target.swiftRuntimeCompatibilityVersion = version
           info.targetVariant?.swiftRuntimeCompatibilityVersion = version
-        } else {
+        } else if (versionString != "none") {
+          // "none" was accepted by the old driver, diagnose other values.
           diagnosticsEngine.emit(
             .error_invalid_arg_value(
               arg: .runtimeCompatibilityVersion, value: versionString))

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -219,6 +219,10 @@ final class SwiftDriverTests: XCTestCase {
   }
   #endif
 
+  func testRuntimeCompatibilityVersion() throws {
+    try assertNoDriverDiagnostics(args: "swiftc", "a.swift", "-runtime-compatibility-version", "none")
+  }
+
   func testInputFiles() throws {
     let driver1 = try Driver(args: ["swiftc", "a.swift", "/tmp/b.swift"])
     XCTAssertEqual(driver1.inputFiles,