[libc++] Move header_information.py up from tests

I need to use header_information.py in a generator script that isn't for tests in an upcoming change. Move it up a level so that it's in utils/libcxx instead of utils/libcxx/tests.

Differential Revision: https://reviews.llvm.org/D157639

# Conflicts:
#	libcxx/test/libcxx/system_reserved_names.gen.py