[LLVM-C] Add an accessor for the kind of a Metadata Node
Summary: Allows for retrieving the type of a metadata node. Has the added benefit of ensuring that the C and C++ kind APIs stay in sync as a failure to add a corresponding LLVMMetadataKind will result in the switch in the accessor being semantically malformed.
Reviewers: whitequark, deadalnix
Reviewed By: whitequark
Differential Revision: https://reviews.llvm.org/D52693