[libc++] P3372R3: constexpr `multimap`

[frederick-vs-ja]

Make multimap constexpr as part of P3372R3.

Note that node-handle::key() is intentionally left not constexpr.

Feature-test macro (also covers constexpr map):

#define __cpp_lib_constexpr_map 202502L // also in <map>

[vinay-deshmukh]

I can take this one, #134330 which adds constexpr for std::map is pretty much done, and in the "final stretch".

std::multimap should really only need the annotations after 134330 goes in.