After a long drought for new material apart from one-off singles, U2 has put out its first substantial collection of new songs since 2017’s “Songs of Experience”: a six-track EP titled “Days of Ash,” not coincidentally being surprise-released on Ash Wednesday. The most somber day on the Christian calendar is occasion for U2 to put out a half-dozen songs with appropriately somber themes — but also, in some cases, a hard-rocking musical settings. The leadoff track is “American Obituary,” a song