Improve CHAR support

Makes minor fixes to the handling of CHAR. This fixes some issues with CHAR types
being promoted to STRING. This is in preparation for IMPALA-1333, but does not
change the behavior of string literals.

Change-Id: I892ffbbb060273e1e83fd336210988a049f7351c
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5345
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins