emit clarifying comment in migrated files

Author: zth

tests/tools_tests/src/migrate/migrated/Migrated_StdlibMigration_Array.res

@@ -1,3 +1,5 @@
+// This file is autogenerated so it can be type checked.
+// It's the migrated version of src/migrate/StdlibMigration_Array.res.
 let shift1 = [1, 2, 3]->Array.shift
 let shift2 = Array.shift([1, 2, 3])
 

tests/tools_tests/test.sh

@@ -46,7 +46,8 @@ done
 for file in src/migrate/StdlibMigration_*.res; do
   expected_file="src/expected/$(basename $file).expected"
   output="src/migrate/migrated/Migrated_$(basename $file)"
-  cp -f "$expected_file" "$output"
+  echo "// This file is autogenerated so it can be type checked.
+// It's the migrated version of $file." > "$output" && cat "$expected_file" >> "$output"
 done
 
 warningYellow='\033[0;33m'