[libc++][Modules] Simplify the __std_clang_module header generation

Post review feedback on D157364. Don't section the __std_clang_module header by macro, put the headers in alphabetical order and repeat the macro guards. Restore header_information.header_restrictions.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D158133

# Conflicts:
#	libcxx/utils/libcxx/header_information.py