Use CallbackVH, instead of WeakVH, to hold MDNode elements.

Use CallbackVH, instead of WeakVH, to hold MDNode elements.
Use FoldingSetNode to unique MDNodes in a context.
Use CallbackVH hooks to update context's MDNodeSet appropriately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80839 91177308-0d34-0410-b5e6-96231b3b80d8