[6.0] Only pass a versioned prebuilt-modules for Mac Catalyst if it exists.

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -419,12 +419,27 @@ public final class DarwinToolchain: Toolchain {
     // doesn't always match the macosx sdk version so the compiler may fail to find
     // the prebuilt module in the versioned sub-dir.
     if frontendTargetInfo.target.triple.isMacCatalyst {
+      let resourceDirPath = VirtualPath.lookup(frontendTargetInfo.runtimeResourcePath.path)
+      let basePrebuiltModulesPath = resourceDirPath.appending(components: "macosx", "prebuilt-modules")
+
+      // Ensure we pass a path that exists. This matches logic used in the Swift frontend.
+      let prebuiltModulesPath: VirtualPath = try {
+        var versionString = sdkInfo.versionString
+        repeat {
+          let versionedPrebuiltModulesPath =
+            basePrebuiltModulesPath.appending(component: versionString)
+          if try fileSystem.exists(versionedPrebuiltModulesPath) {
+            return versionedPrebuiltModulesPath
+          } else if versionString.hasSuffix(".0") {
+            versionString.removeLast(2)
+          } else {
+            return basePrebuiltModulesPath
+          }
+        } while true
+      }()
+
       commandLine.appendFlag(.prebuiltModuleCachePath)
-      commandLine.appendPath(try getToolPath(.swiftCompiler).parentDirectory/*bin*/
-        .parentDirectory/*usr*/
-        .appending(component: "lib").appending(component: "swift")
-        .appending(component: "macosx").appending(component: "prebuilt-modules")
-        .appending(component: sdkInfo.versionString))
+      commandLine.appendPath(prebuiltModulesPath)
     }
 
     // Pass down -clang-target.

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -7029,14 +7029,34 @@ final class SwiftDriverTests: XCTestCase {
         try testInputsPath.appending(component: "mock-sdk.sdk").nativePathString(escaped: false)
 
     do {
-      var driver = try Driver(args: ["swiftc", "-target", "x86_64-apple-ios13.1-macabi", "foo.swift", "-sdk", mockSDKPath],
+      let resourceDirPath: String = try testInputsPath.appending(components: "PrebuiltModules-macOS10.15.xctoolchain", "usr", "lib", "swift").nativePathString(escaped: false)
+
+      var driver = try Driver(args: ["swiftc", "-target", "x86_64-apple-ios13.1-macabi", "foo.swift", "-sdk", mockSDKPath, "-resource-dir", resourceDirPath],
+                              env: envVars)
+      let plannedJobs = try driver.planBuild()
+      let job = plannedJobs[0]
+      XCTAssertTrue(job.commandLine.contains(.flag("-prebuilt-module-cache-path")))
+      XCTAssertTrue(job.commandLine.contains { arg in
+        if case .path(let curPath) = arg {
+          if curPath.basename == "10.15" && curPath.parentDirectory.basename == "prebuilt-modules" && curPath.parentDirectory.parentDirectory.basename == "macosx" {
+              return true
+          }
+        }
+        return false
+      })
+    }
+
+    do {
+      let resourceDirPath: String = try testInputsPath.appending(components: "PrebuiltModules-macOSUnversioned.xctoolchain", "usr", "lib", "swift").nativePathString(escaped: false)
+
+      var driver = try Driver(args: ["swiftc", "-target", "x86_64-apple-ios13.1-macabi", "foo.swift", "-sdk", mockSDKPath, "-resource-dir", resourceDirPath],
                               env: envVars)
       let plannedJobs = try driver.planBuild()
       let job = plannedJobs[0]
       XCTAssertTrue(job.commandLine.contains(.flag("-prebuilt-module-cache-path")))
       XCTAssertTrue(job.commandLine.contains { arg in
         if case .path(let curPath) = arg {
-          if curPath.basename == "10.15" && curPath.parentDirectory.basename == "prebuilt-modules" {
+          if curPath.basename == "prebuilt-modules" && curPath.parentDirectory.basename == "macosx" {
               return true
           }
         }