libstdc++: Add static_assert to std::integer_sequence [PR112473]

C++20 allows class types as non-type template parameters, but
std::integer_sequence explicitly disallows them. Enforce that.

libstdc++-v3/ChangeLog:

	PR libstdc++/112473
	* include/bits/utility.h (integer_sequence): Add static_assert.
	* testsuite/20_util/integer_sequence/112473.cc: New test.