[libc++] Optimize map::insert_or_assign (#155816)

`__emplace_unique` uses `__find_equal`, which can be significantly
faster than `lower_bound`. As a nice side-effect, this also changes the
implementation to the "naive" implementation of trying `insert` first,
and if that fails assign instead. This also matches the
`insert_or_assign` overloads with a hint.

```
Zen 2:
--------------------------------------------------------------------------------------------------------
Benchmark                                                                            old             new
--------------------------------------------------------------------------------------------------------
std::map<int, int>::insert_or_assign(key, value) (already present)/0             1.62 ns         1.53 ns
std::map<int, int>::insert_or_assign(key, value) (already present)/32            5.78 ns         5.99 ns
std::map<int, int>::insert_or_assign(key, value) (already present)/1024          21.5 ns         15.4 ns
std::map<int, int>::insert_or_assign(key, value) (already present)/8192          26.2 ns         20.5 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/0                   22.5 ns         21.1 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/32                  42.9 ns         28.4 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/1024                 118 ns         92.0 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/8192                 227 ns          173 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/0     13.2 ns         18.9 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/32    65.6 ns         39.0 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/1024   127 ns         64.4 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/8192   134 ns         71.4 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/0           45.6 ns         37.3 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/32           142 ns         93.3 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/1024         288 ns          147 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/8192         368 ns          182 ns

Apple M4:
--------------------------------------------------------------------------------------------------------
Benchmark                                                                              old           new
--------------------------------------------------------------------------------------------------------
std::map<int, int>::insert_or_assign(key, value) (already present)/0              0.784 ns      0.740 ns
std::map<int, int>::insert_or_assign(key, value) (already present)/32              2.52 ns       1.77 ns
std::map<int, int>::insert_or_assign(key, value) (already present)/1024            8.72 ns       4.06 ns
std::map<int, int>::insert_or_assign(key, value) (already present)/8192            10.6 ns       3.98 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/0                     17.3 ns       17.2 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/32                    22.5 ns       19.3 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/1024                  56.8 ns       33.5 ns
std::map<int, int>::insert_or_assign(key, value) (new value)/8192                  88.2 ns       41.0 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/0       16.6 ns       11.8 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/32      13.7 ns       30.7 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/1024    46.7 ns       49.1 ns
std::map<std::string, int>::insert_or_assign(key, value) (already present)/8192    41.9 ns       76.9 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/0             40.0 ns       40.5 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/32            38.9 ns       40.0 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/1024          84.9 ns       96.9 ns
std::map<std::string, int>::insert_or_assign(key, value) (new value)/8192           166 ns        149 ns
```