Fix LLVMSetMetadata and LLVMAddNamedMetadataOperand for single value MDNodes

Summary:
MetadataAsValue uses a canonical format that strips the MDNode if it
contains only a single constant value. This triggers an assertion when
trying to cast the value to a MDNode.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7165

llvm-svn: 227319