Fix matching of backreferences in scalar mode

[rctcwyvrn]

.backreference always matched in grapheme semantic mode (using matchSeq() which calls match), making this test fail

    // Backreferences in scalar mode
    // In scalar mode the backreference should not match
    firstMatchTest(#"(.+)\1"#, input: "ée\u{301}", match: "ée\u{301}")
    firstMatchTest(#"(.+)\1"#, input: "ée\u{301}", match: nil, semanticLevel: .unicodeScalar)

Fixed this by adding a bit to the payload for back references to check if it was emitted in unicode scalar mode

[rctcwyvrn]

@swift-ci test

[milseman]

LGTM