BAGHDAD — Anti-government protesters tore down walls and poured into the Iraqi capital’s heavily fortified Green Zone on Saturday, where they stormed parliament in a major escalation of a political crisis that has simmered for months. Supporters of Shiite cleric Muqtada al-Sadr have been holding demonstrations and sit-ins for months to demand an overhaul of Iraq’s corrupt and ineffective political system, but Saturday was the first time they broke into the Green Zone, home to most government ministries and foreign embassies. Iraqi security forces fired tear gas at one entrance of the zone but appeared to be largely standing down as protesters marched through the area, chanting and waving Iraqi flags.