PR libstdc++/83860 avoid dangling references in valarray closure types

PR libstdc++/83860 avoid dangling references in valarray closure types

Store nested closures by value not by reference, to prevent holding
invalid references to temporaries that have been destroyed. This
changes the layout of the closure types, so change their linkage names,
but moving them to a different namespace.

	PR libstdc++/57997
	PR libstdc++/83860
	* include/bits/gslice_array.h (gslice_array): Define default
	constructor as deleted, as per C++11 standard.
	* include/bits/mask_array.h (mask_array): Likewise.
	* include/bits/slice_array.h (slice_array): Likewise.
	* include/bits/valarray_after.h (_GBase, _GClos, _IBase, _IClos): Move
	to namespace __detail.
	(_GBase::_M_expr, _IBase::_M_expr): Use _ValArrayRef for type of data
	members.
	* include/bits/valarray_before.h (_ValArrayRef): New helper for type
	of data members in closure objects.
	(_FunBase, _ValFunClos, _RefFunClos, _UnBase, _UnClos, _BinBase)
	(_BinBase2, _BinBase1, _BinClos, _SBase, _SClos): Move to namespace
	__detail.
	(_FunBase::_M_expr, _UnBase::_M_expr, _BinBase::_M_expr1)
	(_BinBase::_M_expr2, _BinBase2::_M_expr1, _BinBase1::_M_expr2)
	(_SBase::_M_expr): Use _ValArrayRef for type of data members.
	* include/std/valarray (_UnClos, _BinClos, _SClos, _GClos, _IClos)
	(_ValFunClos, _RefFunClos): Move to namespace __detail and add
	using-declarations to namespace std.
	* testsuite/26_numerics/valarray/83860.cc: New.

From-SVN: r259844