[libc++][Modules] Generate the __std_clang_module header

Use header_information to generate the __std_clang_module header. Instead of using lit_header_restrictions like the manually written header did, make a new header_include_requirements to codify what can be included rather than what can be fully tested.

Reviewed By: Mordante, #libc

Differential Revision: https://reviews.llvm.org/D157364

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