MIR Serialization: Serialize the jump table info.

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242357 91177308-0d34-0410-b5e6-96231b3b80d8