[llvm-c] add LLVMReplaceMDNodeOperandWith

I'm working on a tool that visits debug info and massages it using the
llvm-c API. I noticed that LLVMGetOperand special cases MDNodes so I
can get their operands, but I can't replace them. This patch adds
LLVMReplaceMDNodeOperandWith which boils down to
MDNode::replaceOperandWith.

The name was chosen for consistency with LLVMGetMDNodeOperands and
LLVMGetMDNodeNumOperands.

Differential Revision: https://reviews.llvm.org/D136637