IMPALA-1340: removing implicit casts during expr substitution is not always safe

Union statements were sometimes losing necessary casts during
expression substitution, causing the backend union node to receive
slot refs that did not have the same types as the result tuple. Add a
flag to Expr.Substitute() to preserve the root expr types, which adds
back the casts after substitution.

Currently only the union node sets this flag to true, but there may be
other places that are incorrect.

Change-Id: I1b4d9846860ef9694ff0c089f79654b1746d687d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4777
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: Skye Wanderman-Milne <skye@cloudera.com>