Add location of overridden def in error message
Add location of overridden def in error message

When a def is overridden, for example because there are multiple
clauses with the clauses of a different def are between them, it
is nice to know where the def that was overridden starts.

This should fix #1559.

elixir-lang/elixir,kelvinst/elixir,lexmag/elixir,michalmuskala/elixir,kimshrier/elixir,ggcampinho/elixir,antipax/elixir,joshprice/elixir,gfvcastro/elixir,pedrosnk/elixir,pedrosnk/elixir,antipax/elixir,beedub/elixir,lexmag/elixir,kimshrier/elixir,beedub/elixir,ggcampinho/elixir,gfvcastro/elixir,kelvinst/elixir