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

Some clients still have unversioned prebuilt modules, but SwiftDriver assumed
that the versioned path would always exist for macOS (at least for the purposes
of passing it when compiling for Mac Catalyst).

This change duplicates some logic from the Swift frontend to check for the
existence of the versioned macOS prebuilt modules directory before passing it,
optionally stripping off trailing `.0` version components.

This change also updates the driver so that it passes a prebuilt modules directory
relative to the resource directory rather than relative to the compiler binary, also
matching the Swift frontend's behavior.

This addresses <rdar://problem/136047054>.

Author: cwakamo

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -410,12 +410,27 @@ 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.

TestInputs/PrebuiltModules-macOS10.15.xctoolchain/usr/lib/swift/macosx/prebuilt-modules/10.15/.empty-file

@@ -7172,14 +7172,34 @@ 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
           }
         }