DISubprogram metadata have an MDNode representing which base type
contains the vtable pointer for the class as the operand at index
12. Replace the i32 0 with a null MDNode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187970 91177308-0d34-0410-b5e6-96231b3b80d8