[lld] Make tests calling llvm-ar more robust

Some lit tests that call llvm-ar use the 'r' flag. If the target archive
already exists and is in a corrupt state, this can cause the test to fail. We
have added 'rm -f' calls before the llvm-ar calls to increase the
robustness of the tests.

Differential revision: https://reviews.llvm.org/D49184

llvm-svn: 338705