IMPALA-974: Switch from FloatLiteral to DecimalLiteral.

Float/Doubles are lossy so using those as the default literal type
is problematic.

Change-Id: I5a619dd931d576e2e6cd7774139e9bafb9452db9
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2615
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>