[libc] Implement htonl and htons

Per spec:
* https://pubs.opengroup.org/onlinepubs/9699919799/functions/htonl.html
* https://pubs.opengroup.org/onlinepubs/9699919799/functions/htons.html



Also adds UInt16Type and UInt32Type to spec.td

Co-authored-by: Jeff Bailey <jbailey@google.com>

Reviewed By: sivachandra, jeffbailey, rtenneti

Differential Revision: https://reviews.llvm.org/D143795