Add support for reading and writing MessagePack, a binary object serialization
format which aims to be more compact than text formats like JSON or YAML.
The specification can be found at
https://github.com/msgpack/msgpack/blob/master/spec.md
Will be used for encoding metadata in AMDGPU code objects.
Differential Revision: https://reviews.llvm.org/D44429
llvm-svn: 340457
