Fix a file overwrite bug in llvm-ar introduced by changes to

Fix a file overwrite bug in llvm-ar introduced by changes to
createTemporaryFile semantics where it doesn't create a fully unique name
if the basename doesn't exist. This functionality is now optionally
provided by the boolean reuse_current parameter to createTemporaryFile and
makeUnique. The default values differ because of the way these functions
are used in LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18961 91177308-0d34-0410-b5e6-96231b3b80d8