[libc++] Investigate the precondition fix in P3179R9 for existing algorithms

[frederick-vs-ja]

P3179R9 (tracked in #148138 and numerous sub-issues) fixed preconditions for the following algorithms, for both std and ranges versions:

• copy
• copy_if
• move
• transform
• replace_copy
• replace_copy_if
• remove_copy
• remove_copy_if
• unique_copy
• partition_copy
• set_union
• set_intersection
• set_difference
• set_symmetric_difference

I think the precondition fix should be treated as an LWG DR as it probably superseded LWG4262 (as noted in microsoft/STL#5683 (comment)). Perhaps we should investigate whether our precondition checks (if any) are still correct.