Clear the llvm CMake cache before building
Clear the llvm CMake cache before building

Hopefully this will prevent issues where the Jenkins build can get
messed up and stay broken due to switching compilers, etc.

Change-Id: I1638f15a0aecf665189191ee81c6f04cd95d4918
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3780
Reviewed-by: Alex Feinberg <befd6aa234c2f4cc93d21d7577c06d8f75d6f7de@cloudera.com>
Tested-by: jenkins

cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu