[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

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52693

llvm-svn: 343469