clang-tidy: Added an environment variable that allows fix to print the (flutter/engine#37024)

generated diff.

engine/src/flutter/ci/lint.sh

@@ -36,6 +36,15 @@ DART="${DART_BIN}/dart"
 # to have this as an error.
 MAC_HOST_WARNINGS_AS_ERRORS="performance-move-const-arg,performance-unnecessary-value-param"
 
+# FLUTTER_LINT_PRINT_FIX will make it so that fix is executed and the generated
+# diff is printed to stdout if clang-tidy fails. This is helpful for enabling
+# new lints.
+if [[ -z "${FLUTTER_LINT_PRINT_FIX}" ]]; then
+  fix_flag=""
+else
+  fix_flag="--fix"
+fi
+
 COMPILE_COMMANDS="$SRC_DIR/out/host_debug/compile_commands.json"
 if [ ! -f "$COMPILE_COMMANDS" ]; then
   (cd "$SRC_DIR"; ./flutter/tools/gn)
@@ -49,7 +58,18 @@ cd "$SCRIPT_DIR"
   "$SRC_DIR/flutter/tools/clang_tidy/bin/main.dart" \
   --src-dir="$SRC_DIR" \
   --mac-host-warnings-as-errors="$MAC_HOST_WARNINGS_AS_ERRORS" \
-  "$@"
+  $fix_flag \
+  "$@" && true # errors ignored
+clang_tidy_return=$?
+if [ $clang_tidy_return -ne 0 ]; then
+  if [ -n "$fix_flag" ]; then
+    echo "###################################################"
+    echo "# Attempted to fix issues with the following patch:"
+    echo "###################################################"
+    git --no-pager diff
+  fi
+  exit $clang_tidy_return
+fi
 
 echo "$(date +%T) Running pylint"