Remove Driver::UseStdLib.

All but one place are checking options::OPT_nostdlib instead of looking at
this field, so convert that one other place to doing that as well.

No behavior change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308848 91177308-0d34-0410-b5e6-96231b3b80d8