[libc++] [CI] Validate the output of the generated scripts.

This adds a CI job validating that the output of
utils/generate_feature_test_macro_components.py,
libcxx/utils/generate_header_inclusion_tests.py, and
utils/generate_header_tests.py are up to date.

The validation method has been copied from the Format job.

Differential Revision: https://reviews.llvm.org/D99862