Fix code completion tests to use an explicit modules cache path

Fix code completion tests to use an explicit modules cache path

Otherwise the stale module cache data may cause the test to fail.  These
two tests are new and are the only instances of c-index-test with
-fmodules that doesn't have an explicit module cache path.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@242710 91177308-0d34-0410-b5e6-96231b3b80d8