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 archive' calls before the llvm-ar calls to increase the robustness
of the tests.
A similar fix for one test was added in revision r336366.
Also, \llvm\test\Object\ar-error.test relied on the non-existance of the
target archive to cause an intended llvm-ar call failure. A call to 'rm -f' has
been added to ensure that this archive definitely cannot exist.