MIR Serialization: Initial serialization of machine constant pools.

MIR Serialization: Initial serialization of machine constant pools.

This commit implements the initial serialization of machine constant pools and
the constant pool index machine operands. The constant pool is serialized using
a YAML sequence of YAML mappings that represent the constant values.
The target-specific constant pool items aren't serialized by this commit.

Reviewers: Duncan P. N. Exon Smith

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242707 91177308-0d34-0410-b5e6-96231b3b80d8