libstdc++: Improve diagnostics for invalid std::hash specializations [PR115420]

libstdc++: Improve diagnostics for invalid std::hash specializations [PR115420]

When using a key type without a valid std::hash specialization the
unordered containers give confusing diagnostics about the default
constructor being deleted. Add a static_assert that will fail for
disabled std::hash specializations (and for a subset of custom hash
functions).

libstdc++-v3/ChangeLog:

	PR libstdc++/115420
	* include/bits/hashtable.h (_Hashtable): Add static_assert to
	check that hash function is copy constructible.
	* testsuite/23_containers/unordered_map/115420.cc: New test.