YAML I/O - Added default trait support for std:string.  Making another attempt at this, this time doing a clean build on Linux, and running the LLVM, clang, and extra tests, to try to make sure there's no problems.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195134 91177308-0d34-0410-b5e6-96231b3b80d8