[Modules] Fix test to wipe out the cache before using it

This should appease bots:
http://green.lab.llvm.org/green/job/clang-stage1-cmake-RA-incremental_check/35914

rdar://problem/31796737

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@301261 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed