Update for LLVM change (StringSaver)

There is more cleanup to be done here. Once
llvm::sys::Process::GetArgumentVector is switched over to StringSaver,
we can simplify this code a fair amount.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215785 91177308-0d34-0410-b5e6-96231b3b80d8