Fix memory leak in YAML I/O.

Stop using BumpPtrAllocator for HNodes because
they have fields (vector, map) which require HNode 
destructors to be run.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171896 91177308-0d34-0410-b5e6-96231b3b80d8