Find blocklist files from toolchain dir and feed them to compiler

Sources/SwiftDriver/Driver/Driver.swift

@@ -432,6 +432,24 @@ public struct Driver {
     return supportedFrontendFeatures.contains(feature.rawValue)
   }
 
+  @_spi(Testing)
+  public static func findBlocklists(RelativeTo execDir: AbsolutePath) throws ->  [AbsolutePath] {
+    // Expect to find all blocklists in such dir:
+    // .../XcodeDefault.xctoolchain/usr/local/lib/swift/blocklists
+    var results: [AbsolutePath] = []
+    let blockListDir = execDir.parentDirectory
+      .appending(components: "local", "lib", "swift", "blocklists")
+    if (localFileSystem.exists(blockListDir)) {
+      try localFileSystem.getDirectoryContents(blockListDir).forEach {
+        let currentFile = AbsolutePath(blockListDir, try VirtualPath(path: $0).relativePath!)
+        if currentFile.extension == "yml" || currentFile.extension == "yaml" {
+          results.append(currentFile)
+        }
+      }
+    }
+    return results
+  }
+
   /// Handler for emitting diagnostics to stderr.
   public static let stderrDiagnosticsHandler: DiagnosticsEngine.DiagnosticsHandler = { diagnostic in
     stdErrQueue.sync {

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -269,6 +269,13 @@ extension Driver {
       commandLine.appendPath(localPluginPath)
     }
 
+    if isFrontendArgSupported(.blockListFile) {
+      try Driver.findBlocklists(RelativeTo: try toolchain.executableDir).forEach {
+        commandLine.appendFlag(.blockListFile)
+        commandLine.appendPath($0)
+      }
+    }
+
     // Pass down -user-module-version if we are working with a compiler that
     // supports it.
     if let ver = parsedOptions.getLastArgument(.userModuleVersion)?.asSingle,

TestInputs/Dummy.xctoolchain/usr/local/lib/swift/blocklists/block-list1.yml

@@ -0,0 +1 @@
+---

TestInputs/Dummy.xctoolchain/usr/local/lib/swift/blocklists/block-list2.yaml

@@ -0,0 +1 @@
+---

TestInputs/Dummy.xctoolchain/usr/local/lib/swift/blocklists/block-list3.txt

@@ -0,0 +1 @@
+---

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -6822,6 +6822,13 @@ final class SwiftDriverTests: XCTestCase {
 #endif
   }
 
+  func testFindingBlockLists() throws {
+    let execDir = testInputsPath.appending(components: "Dummy.xctoolchain", "usr", "bin")
+    let list = try Driver.findBlocklists(RelativeTo: execDir)
+    XCTAssertEqual(list.count, 2)
+    XCTAssertTrue(list.allSatisfy { $0.extension! == "yml" || $0.extension! == "yaml"})
+  }
+
   func testToolSearching() throws {
 #if os(Windows)
     let PATH = "Path"