[AMDGPU] Update code object metadata format documentation

* Add amdhsa prefix to names to allow other tools to use the metadata
  without collision.
* Make names consistent.
* Simplify structure.
* Change note record ID.
* Switch from YAML to MsgPack format.
* Document metadata assembler directive.

Patch By: t-tye (Tony Tye)
Differential Revision: https://reviews.llvm.org/D53445

llvm-svn: 346992