Fix check failure on hexagon targets.

This test was failing because there is no assembler for hexagon-elf on
this buildbot:

http://lab.llvm.org:8011/builders/clang-hexagon-elf/builds/10470

Fixed by adding -S to the driver invocation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206412 91177308-0d34-0410-b5e6-96231b3b80d8