[libc++] Alphabetize generate_feature_test_macro_components.py. NFCI.

For ease of comparing our list with the official SD-6 list, which is alphabetized.
https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#library-feature-test-macros
This also alphabetizes the lists of headers in which the macros are
defined, which harmlessly alters many comments in <version>.
Also drive-by-fix some trivial flake8 warnings.