StringRef: add DenseMapInfo for StringRef.

Remove the implementation in include/llvm/Support/YAMLTraits.h.
Added a DenseMap type DITypeHashMap in DebugInfo.h:
  DenseMap<std::pair<StringRef, unsigned>, MDNode*>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185852 91177308-0d34-0410-b5e6-96231b3b80d8