Some C9 chords should be Cadd9

[Knute Snortum]

Way back in Lilypond 2.14 if you wrote c:9^7 you would get "Cadd9", but now
you get "C9".  This makes it indistinguishable from c:9.

I propose that c:9^7 should render as "Cadd9".  See code and attached
proposed output.

\version "2.19.15"

\score {
  \new ChordNames \chordmode {
    c1:9^7
    c1:9
  }
  \layout {
  }
}

Knute Snortum
(via Gmail)

c-add9-c9.png
Description: PNG image