stringref because they may not be nul terminated. For options like -Lfoo
this now avoids a O(n) temporary std::strings where N is the length of
the string after -L.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82345 91177308-0d34-0410-b5e6-96231b3b80d8