Revert "[Option] Store arg strings in a set backed by a BumpPtrAllocator"

This broke clang's crash-report.c test, and I haven't been able to
figure it out yet.

This reverts commit r186319.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186329 91177308-0d34-0410-b5e6-96231b3b80d8