Surely Diana Damrau sings the highest of any soprano. In a world where high “C” is considered an upper range, she goes higher. Much higher. “Last December, in the first modern performance of Antonio Salieri’s “Europa Riconosciuta,” at La Scala in Milan, she topped the composer’s three F sharps with an unwritten high G. What’s more, she managed to give that unearthly note real sheen and body. And in concert, she has sung an A flat in Johann Strauss’s “Voices of Spring.”