Add MCInst annotation mechanism to MCInstrAnalysis class.

Summary:
Add three new MCOperand types: Annotation, LandingPad and GnuArgsSize.

Annotation is used for associating random data with MCInsts.  Clients can
construct their own annotation types (subclassed from MCAnnotation) and
associate them with instructions.  Annotations are looked up by string keys.

Annotations can be added, removed and queried using an instance of the
MCInstrAnalysis class.

The LandingPad operand is a MCSymbol, uint64_t pair used to encode exception
handling information for call instructions.

GnuArgsSize is used to annotate calls with the DW_CFA_GNU_args_size attribute.

(cherry picked from FBD3597877)