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
llvm-svn: 242707