Introduce a command line option to enable statement frontiers, enabled
by default in optimized builds with DWARF2+ debug information.

This patch depends on an earlier patch that completed the
infrastructure for debug markers, and on another patch that turns -g
into a negatable option prefix.

for  gcc/ChangeLog

	* common.opt (gstatement-frontiers): New, setting
	debug_nonbind_markers_p.
	* rtl.h (MAY_HAVE_DEBUG_MARKER_INSNS): Activate.
	* toplev.c (process_options): Autodetect value for debug statement
	frontiers option.
	* tree.h (MAY_HAVE_DEBUG_MARKER_STMTS): Activate.
	* doc/invoke.texi (gstatement-frontiers, gno-statement-frontiers): New.

From-SVN: r255569