[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

llvm-svn: 341085