generate_header_tests.py: Sort the header files ASCIIbetically.

Otherwise they come out in random (inode?) order.

Also `chmod +x` the generator, and re-run it. Somehow on Marek's
machine it produced \r\n line endings?! Open all files with
`newline='\n'` so that (if the Python3 docs are correct)
that won't happen again.

Differential Revision: https://reviews.llvm.org/D93137