[libc++] Improve generate_feature_test_macro_components.py.

This improves the naming of the fields `depends`/`internal_depends`. It
also adds the documentation for this script. The changes are based on
D99290 and its review comments.

Differential Revision: https://reviews.llvm.org/D99615

GitOrigin-RevId: c2c68a5940dcd9c9e910dad78b96ecdec8517011
1 file changed