Put PLATFORM_DIR plugin paths before SDK-path-derived paths

Since PLATFORM_DIR is the override, put it before the default-derived path.

Author: DougGregor

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -456,24 +456,25 @@ public final class DarwinToolchain: Toolchain {
     }
 
     if driver.isFrontendArgSupported(.externalPluginPath) {
-      // Determine the platform path based on the SDK path.
-      addPluginPaths(
-        forPlatform: VirtualPath.lookup(sdkPath)
-          .parentDirectory
-          .parentDirectory
-          .parentDirectory,
-        commandLine: &commandLine
-      )
-
       // If the PLATFORM_DIR environment variable is set, also add plugin
-      // paths into it.
+      // paths into it. Since this is a user override, it comes beore the
+      // default platform path that's based on the SDK.
       if let platformDir = env["PLATFORM_DIR"],
          let platformPath = try? VirtualPath(path: platformDir) {
         addPluginPaths(
           forPlatform: platformPath,
           commandLine: &commandLine
         )
       }
+
+      // Determine the platform path based on the SDK path.
+      addPluginPaths(
+        forPlatform: VirtualPath.lookup(sdkPath)
+          .parentDirectory
+          .parentDirectory
+          .parentDirectory,
+        commandLine: &commandLine
+      )
     }
   }
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -7805,7 +7805,7 @@ final class SwiftDriverTests: XCTestCase {
     XCTAssertNotNil(envPlatformPluginPathIndex)
 
     if let platformPluginPathIndex, let envPlatformPluginPathIndex {
-      XCTAssertLessThan(platformPluginPathIndex, envPlatformPluginPathIndex)
+      XCTAssertLessThan(envPlatformPluginPathIndex, platformPluginPathIndex)
     }
 
     let toolchainPluginPathIndex = job.commandLine.firstIndex(of: .path(.absolute(try driver.toolchain.executableDir.parentDirectory.appending(components: "lib", "swift", "host", "plugins"))))