Give better source location info for Const/Write errors.

[benl]

Try and find the write effect / source of constness. or at least give different source locations.

 test/Error/Purity/ConstWrite/Main.ds:16:8
    Conflicting region constraints.
              constraint: Base.Mutable %86
         from the use of: a
                      at: test/Error/Purity/ConstWrite/Main.ds:16:8

     conflicts with,
              constraint: Base.Const %86
         from the use of: a
                      at: test/Error/Purity/ConstWrite/Main.ds:16:8