[ThinLTO] Fix bot failures from r333335

Change value in vector from StringRef to std::string to avoid errors
when trying to initialize from a std::string.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333336 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed