Revert "Switch debug builds to use clang."

This reverts commit e190eb50315d9c9c813fd04fbbb9713afabf8c14.

Change-Id: I2089caddbc902a2e90a0d42002b75a2d172abb45
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3306
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>