PARIS — The reopening of the Notre Dame Cathedral should have been a victory lap for French President Emmanuel Macron. Instead, the celebrations are shadowed by calls for him to step down after Prime Minister Michel Barnier handed in his resignation on Thursday, following a no-confidence vote that collapsed his government. 

Barnier’s fragile parliamentary coalition was the first in more than six decades to fall apart in France, the second-biggest economic power in the European Union….