Revert "Tooling/ReplacementsYaml.h: Prune Tooling's llvm::yaml::ScalarTraits<std::string> corresponding to LLVM r193344."

This reverts commit r193350, because it addressed an llvm API change that has been reverted.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193353 91177308-0d34-0410-b5e6-96231b3b80d8