remove temporal file directory before testing in thinlto-debug-fission.ll

Summary:
This removes the %T/dwo directory before calling ld.lld in ELF/lto/thinlto-debug-fission.ll so that
files aren't left over from previous runs.

Reviewers: espindola, pcc

Reviewed By: pcc

Subscribers: inglorion, emaste, arichardson, eraman, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49390