MIR Serialization: Serialize the jump table info.

The jump table info is serialized using a YAML mapping that contains its kind
and a YAML sequence of jump table entries. A jump table entry is a YAML mapping
that has an ID and an inline YAML sequence of machine basic block references.

The testcase 'CodeGen/MIR/X86/jump-table-info.mir' doesn't have any instructions
because one of them contains a jump table index operand. The jump table index
operands will be serialized in a follow up patch, and the appropriate
instructions will be added to this testcase.

Reviewers: Duncan P. N. Exon Smith
llvm-svn: 242357