[MsgPack] New MsgPackDocument class

Summary:
A class that exposes a simple in-memory representation of a document of
MsgPack objects, that can be read from and written to MsgPack, read from
and written to YAML, and inspected and modified in memory. This is
intended to be a lighter-weight (in terms of memory allocations)
replacement for MsgPackTypes.

Two subsequent changes will:
1. switch AMDGPU HSA metadata to using MsgPackDocument instead of
   MsgPackTypes;
2. add MsgPack AMDGPU PAL metadata via MsgPackDocument.

Differential Revision: https://reviews.llvm.org/D57023

Change-Id: Ie15a054831d5a6467c5867c064c8f8f6b80270e1
llvm-svn: 356080