[libc++] Improve the output of the generated-output CI job (#68903)

The step that checked for ignore_format.txt being consistent with the
tree wouldn't print any explicit diagnostic when failing, which led to
confusion. After this patch, an explicit diagnostic will be printed by
the job along with the required diff to ignore_format.txt.

Author: ldionne

libcxx/utils/ci/run-buildbot

@@ -209,6 +209,8 @@ check-generated-output)
     clean
     generate-cmake
 
+    set +x # Printing all the commands below just creates extremely confusing output
+
     # Reject patches that forgot to re-run the generator scripts.
     echo "+++ Making sure the generator scripts were run"
     ${NINJA} -vC "${BUILD_DIR}" libcxx-generate-files
@@ -222,20 +224,23 @@ check-generated-output)
         false
     fi
 
+    echo "+++ Making sure libcxx/utils/data/ignore_format.txt was updated appropriately"
+    cp ${MONOREPO_ROOT}/libcxx/utils/data/ignore_format.txt ${BUILD_DIR}/before.txt
     ${MONOREPO_ROOT}/libcxx/utils/generate_ignore_format.sh
-    git diff | tee ${BUILD_DIR}/generated_output.patch
-    git ls-files -o --exclude-standard | tee ${BUILD_DIR}/generated_output.status
-    ! grep -q '^--- a' ${BUILD_DIR}/generated_output.patch || false
-    if [ -s ${BUILD_DIR}/generated_output.status ]; then
+    diff ${BUILD_DIR}/before.txt ${MONOREPO_ROOT}/libcxx/utils/data/ignore_format.txt | tee ${BUILD_DIR}/ignore_format.diff || true
+    if [ -s ${BUILD_DIR}/ignore_format.diff ]; then
         echo "It looks like the list of not formatted files has changed."
         echo "If a file is now properly formatted with clang-format, remove the file name from "
         echo "libcxx/utils/data/ignore_format.txt. Otherwise you have to fix the"
-        echo "formatting of some of the changed files."
+        echo "formatting of some of the changed files. The diff above represents the "
+        echo "changes that would be needed to ignore_format.txt to keep it representative "
+        echo "of which files are mis-formatted in the project."
         false
     fi
 
     # Reject patches that introduce non-ASCII characters or hard tabs.
     # Depends on LC_COLLATE set at the top of this script.
+    set -x
     ! grep -rn '[^ -~]' libcxx/include libcxx/src libcxx/test libcxx/benchmarks \
            --exclude '*.dat' \
            --exclude '*unicode*.cpp' \