* operators.def: Add *_FOLD_EXPR.
	* cp-tree.h (FOLD_EXPR_P): Parenthesize.
	* mangle.c (write_expression): Handle fold-expressions.
	* pt.c (tsubst_unary_left_fold, tsubst_binary_left_fold)
	(tsubst_unary_right_fold, tsubst_binary_right_fold): Handle
	partial instantiation.

From-SVN: r238390