DWARF: fix stack usage assessment for DW_OP_neg

DWARF: fix stack usage assessment for DW_OP_neg

When the DWARF back-end generates DW_OP_neg operations in DWARF
procedures, we get an ICE because of inconsistent stack usage
computation for the embedding expression. This is because
resolve_args_picking_1 thinks DW_OP_neg is a binary operation (pops 2
stack slots, pushes 1) whereas it really is an unary one (one pop, one
push).

This change fixes resolve_args_picking_1 and adds a regression testcase
(which crashes with the current trunk).  Bootstrapped and regtested
without regression on x86_64-linux.

gcc/

	* dwarf2out.c (resolve_args_picking_1): Consider DW_OP_neg as an
	unary operation, not a binary one.

gcc/testsuite/

	* gnat.dg/debug6.adb, gnat.dg/debug6_pkg.ads: New testcase.

From-SVN: r236087