YAMLTraits.h: replace DenseMap that used a bad implementation of DenseMapInfo
for StringRef with a StringMap

The bug is that the empty key compares equal to the tombstone key.

Also added an assertion to DenseMap to catch similar bugs in future.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187866 91177308-0d34-0410-b5e6-96231b3b80d8