LWG3816: `flat_map` and `flat_multimap` should impose sequence container requirements

[ldionne]

Link: https://wg21.link/LWG3816

[frederick-vs-ja]

This looks like pure wording correction without which we can't properly implement flat_(multi)map. Perhaps it should be considered implemented in LLVM 20 together with flat_(multi)map.

[ldionne]

It looks like we don't check that the iterator is a Cpp17RandomAccessIterator -- should we be checking for that?

CC @huixie90