This is not available in the C API, so add functions to extra.cpp:
- `LLVMMetaDataGetOperand` wraps MDNode::getOperand
- `LLVMMetaDataGetNumOperands` wraps MDNode::getNumOperands
This is mainly useful to get access to top-level debug information through 'llvm.dbg.cu'.
Wraps LLVM C-API functions `LLVMGetNamedMetadataNumOperands`, `LLVMGetNamedMetadataOperands` and `LLVMAddNamedMetadataOperand`.
Also add testcase.
- property z_ext_value gets the zero-extended value using LLVMConstIntGetZExtValue
- property s_ext_value gets the sign-extended value using LLVMConstIntGetSExtValue
- add test case for both