Cover all the constructor names in GADT

[jinwoo]

Currently only the first constructor name is handled: https://github.com/google/haskell-indexer/blob/master/haskell-indexer-backend-ghc/src/Language/Haskell/Indexer/Backend/Ghc.hs#L327

[awpr]

Civilian's note: this appears to refer only to multiple constructors declared by the same ConDecl, rather than multiple ConDecls within a single HsDataDefn, e.g. we'd omit nodes/edges for B and C in the following:

data MyType where A, B, C :: MyType