IMPALA-1429: Avoid ambiguous col refs after subquery rewrite

Prior to this commit, unqualified column refs may become ambiguous
during the rewriting of subqueries. As part of rewrite, the subquery is
transformed into an inline view and since inline views provide
candidates during column resolution, there may be multiple candidates
after the rewrite.

This commit adds a layer to separate column labels exposed by an inline
view from column labels used to construct the query within the inline
view.

Example:
  SELECT 1 FROM alltypes WHERE int_col IN (SELECT 1 AS int_col)

  would have been rewritten as

  SELECT 1
  FROM alltypes
  SEMI JOIN (SELECT 1 AS int_col) `$a` ON `$a`.int_col = int_col

  making the final int_col ambiguous. The rewrite is now

  ...
  SEMI JOIN (SELECT 1 AS int_col) `$a` (`$c1`) ON `$a`.`$c1` = int_col

The ability to override column labels is not available to users.

Change-Id: I9be0a331b7dbf24e4d66dc44ec109042d7250ed9
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5230
Reviewed-by: Casey Ching <casey@cloudera.com>
Tested-by: jenkins