Introducing LLVMMetadataRef

Summary:
This seems like an uncontroversial first step toward providing access to the metadata hierarchy that now exists in LLVM. This should allow for good debug info support from C.

Future plans are to deprecate API that take mixed bags of values and metadata (mainly the LLVMMDNode family of functions) and migrate the rest toward the use of LLVMMetadataRef.

Once this is in place, mapping of DIBuilder will be able to start.

Reviewers: mehdi_amini, echristo, whitequark, jketema, Wallbraker

Reviewed By: Wallbraker

Subscribers: Eugene.Zelenko, axw, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D19448

llvm-svn: 300447