[libc++] Module fixes for __synth_three_way.

These changes make it possible to use __synth_three_way in modules. The
change from a lambda to a function is a Clang issue.

The change is list was needed since the compiler couldn't deduce the
comparison template argument.

Adds a few missing includes too.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D146545