[BOLT][Refactoring] Isolate changes to MC layer

Summary:
Changes that we made to MCInst, MCOperand, MCExpr, etc. are now all
moved into tools/llvm-bolt. That required a change to the way we handle
annotations and any extra operands for MCInst.

Any MCPlus information is now attached via an extra operand of type
MCInst with an opcode ANNOTATION_LABEL. Since this operand is MCInst, we
attach extra info as operands to this instruction. For first-level
annotations use functions to access the information, such as
getConditionalTailCall() or getEHInfo(), etc. For the rest, optional or
second-class annotations, use a general named-annotation interface such
as getAnnotationAs<uint64_t>(Inst, "Count").

I did a test on HHVM binary, and a memory consumption went down a little
bit while the runtime remained the same.

(cherry picked from FBD7405412)