clangd_check: write .clangd as part of test (#54766)

This eliminates the checked-in `.clangd` file at the root of the engine repository, which gets picked up by editors that rely on `clangd`. In particular, the `Remove: [-m*, -f*]` is problematic for iOS/macOS embedder developers since it removes the `-fobjc-arc` flag, which causes `clangd` to emit errors when using ARC features. Example:

    Cannot create __weak reference in file using manual reference counting

This removes the checked-in file, and instead writes the file before running the test, then cleans it up afterwards. Unfortunately, `clangd` does not appear to have a mechanism to point to a config file elsewhere and run as though it were present in a specified directory.

Fixes: flutter/flutter#154064

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

.clangd

@@ -45158,7 +45158,6 @@ ORIGIN: ../../../flutter/vulkan/vulkan_utilities.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/vulkan/vulkan_window.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/vulkan/vulkan_window.h + ../../../flutter/LICENSE
 TYPE: LicenseType.bsd
-FILE: ../../../flutter/.clangd
 FILE: ../../../flutter/.engine-release.version
 FILE: ../../../flutter/.pylintrc
 FILE: ../../../flutter/assets/asset_manager.cc

ci/licenses_golden/licenses_flutter

@@ -106,8 +106,21 @@ void main(List<String> args) {
     return;
   }
 
-  // Run clangd.
+  final Engine engineRoot = Engine.findWithin(p.canonicalize(compileCommandsDir));
+  final io.File clangdConfig = io.File(p.join(engineRoot.flutterDir.path, '.clangd'));
   try {
+    // Write a .clangd file to the engine root directory.
+    //
+    // See:
+    // - https://clangd.llvm.org/config#compileflags
+    // - https://github.com/clangd/clangd/issues/662
+    clangdConfig.writeAsStringSync(
+      'CompileFlags:\n'
+      '  Add: -Wno-unknown-warning-option\n'
+      '  Remove: [-m*, -f*]\n'
+    );
+
+    // Run clangd.
     final io.ProcessResult result = io.Process.runSync(clangd, <String>[
       '--compile-commands-dir',
       compileCommandsDir,
@@ -128,5 +141,8 @@ void main(List<String> args) {
     io.stderr.writeln('Failed to run clangd: $e');
     io.stderr.writeln(const JsonEncoder.withIndent('  ').convert(entry));
     io.exitCode = 1;
+  } finally {
+    // Remove the copied .clangd file from the engine root directory.
+    clangdConfig.deleteSync();
   }
 }