libstdc++: pstl: port away from is_trivial

In preparation for the deprecation of is_trivial (P3247R2).
Unfortunately I am unable to fully understand what aspect of triviality
seems to matter for these algorithms, so I just ported is_trivial to its
direct equivalent (trivially copyable + trivially default
constructible.)

libstdc++-v3/ChangeLog:

	* include/pstl/algorithm_impl.h (__remove_elements): Port away
	from is_trivial.
	(__pattern_inplace_merge): Likewise.
	* include/pstl/glue_memory_impl.h (uninitialized_copy): Likewise.
	(uninitialized_copy_n): Likewise.
	(uninitialized_move): Likewise.
	(uninitialized_move_n): Likewise.
	(uninitialized_default_construct): Likewise.
	(uninitialized_default_construct_n): Likewise.
	(uninitialized_value_construct): Likewise.
	(uninitialized_value_construct_n): Likewise.
	* testsuite/20_util/specialized_algorithms/pstl/uninitialized_construct.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/pstl/uninitialized_copy_move.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/pstl/uninitialized_fill_destroy.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/partition.cc:
	Likewise.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>