The _Tuple_impl constructor for allocator-extended construction from a
different tuple type uses the _Tuple_impl's own _Head type in the
__use_alloc test. That is incorrect, because the argument tuple could
have a different type. Using the wrong type might select the
leading-allocator convention when it should use the trailing-allocator
convention, or vice versa.

libstdc++-v3/ChangeLog:

	PR libstdc++/96803
	* include/std/tuple
	(_Tuple_impl(allocator_arg_t, Alloc, const _Tuple_impl<U...>&)):
	Replace parameter pack with a type parameter and a pack and pass
	the first type to __use_alloc.
	* testsuite/20_util/tuple/cons/96803.cc: New test.

Tested powerpc64le-linux. Committed to trunk.

This isn't a regression, but would be safe to backport and fixes a
conformance bug, albeit one that nobody has ever noticed.