Currently C/C++ flags like $(GENDIR) are not expanded when passed to bazel_clang_tidy: https://parsley.mongodb.com/evergreen/mongodb_mongo_master_amazon_linux2_arm64_dynamic_compile_run_bazel_clang_tidy_patch_54b4b9ea3c33c7a345ef44aa507c87bc49589da5_66a4367e6e1bdb00076283fc_24_07_26_23_51_30/0/task?bookmarks=0%2C2485&selectedLineRange=L1551&shareLine=1551

Since we now rely on GENDIR expanding, this is required to make clang tidy work with generated source code or code that relies on generated source code.

Call https://bazel.build/rules/lib/builtins/ctx#expand_make_variables to expand the flags passed in