modify declare-use.S to work when full toolchain not present

Test doesn't actually require production of an object file and for
some targets (e.g. hexagon) an assembler is not always available when
lit tests are run.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204144 91177308-0d34-0410-b5e6-96231b3b80d8