This commit adds a new abstraction that a target can register with
TargetRegistry to disassemble an ELF note record. This is different to
the ELFNoteDumper abstraction I added in an earlier change; this new one
takes an MCTargetStreamer to stream directives into, and is intended for
a disassembler that wants to show the note record in a form that is
editable and re-assemblable.
A subsequent commit will add an implementation of this in AMDGPU.
There is currently no in-tree user of this.