Implement module-independent data retrieval operations for LLVMNamedMDNodeRef

[junlarsen]

I'd like to expand the LLVM C API, more specifically the Modules/NamedMDNode API as it has a rather interesting way to retrieve data owned by a LLVMNamedMDNodeRef

In the C++ API, all the data associated with a llvm::NamedMDNode is owned by the node itself so it would only make sense to pull said data from the node itself. For unknown? reasons [1], the C API requires the owning module of a LLVMNamedMDNodeRef to retrieve the data. Looking closer at the implementation we can tell that we shouldn't need the module to retrieve parts of this data.

The functions I have added in this patch have the same functionality as the ones present in the C API. The only difference is that we don't need to provide a module to look up a LLVMNamedMDNodeRef inside of, instead we provide the instance returned by LLVMGetNamedMetadata or LLVMGetOrInsertNamedMetadata.

[1]: I imagine it was designed this way because in the C++ API, llvm::NamedMDNodes may be detached and not associated with a Module so providing the owning module was the natural choice, but this is not possible to do through the C API alone which means we can do better.

[saudet]

BTW, would you be able to help this guy? https://stackoverflow.com/questions/65634369/llvm-ir-with-java-api