[libc++] Optimize vector growing of trivially relocatable types (#76657)

This patch introduces a new trait to represent whether a type is
trivially
relocatable, and uses that trait to optimize the growth of a std::vector
of trivially relocatable objects.

```
--------------------------------------------------
Benchmark                           old        new
--------------------------------------------------
bm_grow<int>                    1354 ns    1301 ns
bm_grow<std::string>            5584 ns    3370 ns
bm_grow<std::unique_ptr<int>>   3506 ns    1994 ns
bm_grow<std::deque<int>>       27114 ns   27209 ns
```

This also changes to order of moving and destroying the objects when
growing the vector. This should not affect our conformance.