Fixes #8799

Motivation / Problem

libc++ says boom when opening multiplayer window and click "find servers".

Description

In other words, it should only (!) return true if A comes for B.
This promise was broken for the situation where two values are
identical. It would return true in these cases too. This is of
course not possible: if two values are identical, neither come
before the other. As such, the sorter was not imposing strict
weak ordering relations.

libstdc++ handled this scenario just fine, but libc++ crashes
badly on this, as it allowed comparing of [begin, end] instead
of [begin, end).
libc++ considered this not a bug (and by specs, they are correct;
just this way of crashing is of course a bit harsh):
https://bugs.llvm.org/show_bug.cgi?id=47903

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker