[LLVM-C] Add Bindings For Named Metadata
Summary: Add a new type for named metadata nodes. Use this to implement iterators and accessors for NamedMDNodes and extend the echo test to use them to copy module-level debug information.
Reviewers: whitequark, deadalnix, aprantl, dexonsmith
Reviewed By: whitequark
Subscribers: Wallbraker, JDevlieghere, llvm-commits, harlanhaskins
Differential Revision: https://reviews.llvm.org/D47179