Only reference Alloc swap if permitted by AllocTraits

[BurningEnlightenment]

Describe the bug The following code fails to compile with std::pmr::polymorphic_allocator or any allocator without a swap implementation: https://github.com/greg7mdp/parallel-hashmap/blob/f5e1638a912cdb83fde49ed4611acf9832b54586/parallel_hashmap/phmap.h#L1660-L1665

To Reproduce Compile the following code with C++17 or higher

#include <memory_resource>
#include <utility>

#include <parallel_hashmap/phmap.h>

template <typename K, typename T>
using pmrmap = phmap::flat_hash_map<K, T, std::hash<K>, std::equal_to<>, std::pmr::polymorphic_allocator<std::pair<const K, T>>>;

int main()
{
    pmrmap<int, int> m1;
    pmrmap<int, int> m2;
    using std::swap;
    swap(m1, m2);
}

Expected behavior No compiler errors (the UB in case of mismatching allocators is fine, however, in this case we do have matching allocators).

Environment	OS	Compiler	project version
Windows 10	MSVC 19.35.32216.1	v1.3.8
Ubuntu 22.04.1 LTS	GCC 11.3.0	v1.3.8

Additional context This is sort of a continuation of #128.

[greg7mdp]

Thanks for the report, @BurningEnlightenment , issue should be fixed now. I'm closing it, but feel free to reopen if not fully fixed.