libstdc++: Synchronize PSTL with upstream

libstdc++: Synchronize PSTL with upstream

This patch rebases the C++17 parallel algorithms implementation (pstl)
against the current upstream version, commit 843c12d6a.

This version does not currently include the recently added OpenMP
backend, that will be considered for a future version.

libstdc++-v3/ChangeLog:
	* include/pstl/algorithm_fwd.h: Synchronize with upstream.
	* include/pstl/algorithm_impl.h: Likewise.
	* include/pstl/execution_defs.h: Likewise.
	* include/pstl/execution_impl.h: Likewise.
	* include/pstl/glue_algorithm_impl.h: Likewise.
	* include/pstl/glue_execution_defs.h: Likewise.
	* include/pstl/glue_memory_impl.h: Likewise.
	* include/pstl/glue_numeric_impl.h: Likewise.
	* include/pstl/memory_impl.h: Likewise.
	* include/pstl/numeric_fwd.h: Likewise.
	* include/pstl/numeric_impl.h: Likewise.
	* include/pstl/parallel_backend.h: Likewise.
	* include/pstl/parallel_backend_serial.h: Likewise.
	* include/pstl/parallel_backend_tbb.h: Likewise.
	* include/pstl/parallel_impl.h: Likewise.
	* include/pstl/pstl_config.h: Likewise.
	* include/pstl/unseq_backend_simd.h: Likewise.
	* include/pstl/utils.h: 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_merge/inplace_merge.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_merge/merge.cc: Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/copy_if.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/copy_move.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/fill.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/generate.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/is_partitioned.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/partition.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/partition_copy.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/remove.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/remove_copy.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/replace.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/replace_copy.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/rotate.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/rotate_copy.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/swap_ranges.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/transform_binary.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/transform_unary.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/unique.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_modifying_operations/unique_copy_equal.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/adjacent_find.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/all_of.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/any_of.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/count.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/equal.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/find.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/find_end.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/find_first_of.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/find_if.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/for_each.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/mismatch.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/none_of.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/nth_element.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/reverse.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/reverse_copy.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_nonmodifying/search_n.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/includes.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/is_heap.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/is_sorted.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/lexicographical_compare.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/minmax_element.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/partial_sort.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/partial_sort_copy.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/set.cc:
	Likewise.
	* testsuite/25_algorithms/pstl/alg_sorting/sort.cc:
	Likewise.
	* testsuite/26_numerics/pstl/numeric_ops/adjacent_difference.cc:
	Likewise.
	* testsuite/26_numerics/pstl/numeric_ops/reduce.cc:
	Likewise.
	* testsuite/26_numerics/pstl/numeric_ops/scan.cc:
	Likewise.
	* testsuite/26_numerics/pstl/numeric_ops/transform_reduce.cc:
	Likewise.
	* testsuite/26_numerics/pstl/numeric_ops/transform_scan.cc:
	Likewise.
	* testsuite/util/pstl/test_utils.h:
	Likewise.