libstdc++: [_Hashtable] Enhance performance benches

Add benches on insert with hint and before begin cache.

libstdc++-v3/ChangeLog:

	* testsuite/performance/23_containers/insert/54075.cc: Add lookup on unknown entries
	w/o copy to see potential impact of memory fragmentation enhancements.
	* testsuite/performance/23_containers/insert/unordered_multiset_hint.cc: Enhance hash
	functor to make it perfect, exactly 1 entry per bucket. Also use hash functor tagged as
	slow or not to bench w/o hash code cache.
	* testsuite/performance/23_containers/insert/unordered_set_hint.cc: New test case. Like
	previous one but using std::unordered_set.
	* testsuite/performance/23_containers/insert/unordered_set_range_insert.cc: New test case.
	Check performance of range-insertion compared to individual insertions.
	* testsuite/performance/23_containers/insert_erase/unordered_small_size.cc: Add same bench
	but after a copy to demonstrate impact of enhancements regarding memory fragmentation.