Fix usage of flat_hash_map with move only keys

[gnusi]

This PR fixes compilation of flat_hash_map with move only keys, e.g. the following code won't compile:

absl::flat_hash_map<std::unique_ptr<int>, int> a;
absl::flat_hash_map<std::unique_ptr<int>, int> b;
b = std::move(a); // causes compilation error

The reason is that compiler tries to compile a wrong branch due to some values and if's aren't marked with constexpr.

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[derekmauro]

Could you also add a test case like the one in the example to prevent a regression?

[derekmauro]

This is now producing a warning:

./absl/container/internal/container_memory.h:434:35: error: parameter 'alloc' set but not used [-Werror=unused-but-set-parameter]
  434 |   static auto transfer(Allocator* alloc, slot_type* new_slot,

It seems that whatever analysis GCC is doing doesn't understand constexpr branches.

I'm guessing [[maybe_unused]] will fix this. Or perhaps kIsRelocatable could be a bool and the std::conjunction should use ::value instead of ::type(), maybe that would also give GCC a hint?

[gnusi]

Let me try, I'm going to also add a test case as requested.