[MC] Use a variant to hold MCCFIInstruction state (NFC) (llvm#164720) (#682)

AMDGPU requires more complex CFI rules, normally these would be
expressed with .cfi_escape, however this would make the CFI unreadable
and makes it difficult to update registers in CFI instructions (also
something AMDGPU requires).

Authored-by: Emma Pilkington <[email protected]>

(cherry picked from commit fd94b41)

Author: slinder1