[libc++] Optimize __hash_table copy constructors and assignment (#151951) ``` ---------------------------------------------------------------------------------------------------------------------- Benchmark old new ---------------------------------------------------------------------------------------------------------------------- std::unordered_set::ctor(const&)/0 15.4 ns 14.6 ns std::unordered_set::ctor(const&)/32 686 ns 322 ns std::unordered_set::ctor(const&)/1024 35839 ns 21490 ns std::unordered_set::ctor(const&)/8192 385790 ns 280270 ns std::unordered_set::operator=(const&) (into cleared Container)/0 15.1 ns 15.9 ns std::unordered_set::operator=(const&) (into cleared Container)/32 1077 ns 333 ns std::unordered_set::operator=(const&) (into cleared Container)/1024 31296 ns 9984 ns std::unordered_set::operator=(const&) (into cleared Container)/8192 266776 ns 109418 ns std::unordered_set::operator=(const&) (into partially populated Container)/0 15.1 ns 16.3 ns std::unordered_set::operator=(const&) (into partially populated Container)/32 962 ns 320 ns std::unordered_set::operator=(const&) (into partially populated Container)/1024 31713 ns 10128 ns std::unordered_set::operator=(const&) (into partially populated Container)/8192 266113 ns 108525 ns std::unordered_set::operator=(const&) (into populated Container)/0 0.990 ns 2.03 ns std::unordered_set::operator=(const&) (into populated Container)/32 963 ns 263 ns std::unordered_set::operator=(const&) (into populated Container)/1024 27600 ns 7793 ns std::unordered_set::operator=(const&) (into populated Container)/8192 235295 ns 66248 ns std::unordered_set::ctor(const&)/0 16.0 ns 15.0 ns std::unordered_set::ctor(const&)/32 2950 ns 1277 ns std::unordered_set::ctor(const&)/1024 246935 ns 73762 ns std::unordered_set::ctor(const&)/8192 3310895 ns 2468608 ns std::unordered_set::operator=(const&) (into cleared Container)/0 16.1 ns 15.8 ns std::unordered_set::operator=(const&) (into cleared Container)/32 5856 ns 1039 ns std::unordered_set::operator=(const&) (into cleared Container)/1024 170436 ns 74836 ns std::unordered_set::operator=(const&) (into cleared Container)/8192 1574235 ns 1096891 ns std::unordered_set::operator=(const&) (into partially populated Container)/0 16.0 ns 16.3 ns std::unordered_set::operator=(const&) (into partially populated Container)/32 5571 ns 1064 ns std::unordered_set::operator=(const&) (into partially populated Container)/1024 199220 ns 75462 ns std::unordered_set::operator=(const&) (into partially populated Container)/8192 1552465 ns 1116094 ns std::unordered_set::operator=(const&) (into populated Container)/0 1.70 ns 2.14 ns std::unordered_set::operator=(const&) (into populated Container)/32 2562 ns 645 ns std::unordered_set::operator=(const&) (into populated Container)/1024 228608 ns 39100 ns std::unordered_set::operator=(const&) (into populated Container)/8192 2013723 ns 390401 ns ``` Fixes #77657

[D121492] [libc++] Fix performance inconsistency between unordered_map copy-assignment and copy-construction #77657

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