move ElementVH out of the MDNode class into the MDNode.cpp file. Among

move ElementVH out of the MDNode class into the MDNode.cpp file.  Among
other things, this avoids vtable and rtti data for it being splatted in
every translation unit that uses it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92207 91177308-0d34-0410-b5e6-96231b3b80d8