MIR Serialization: Serialize the fixed stack objects.

This commit serializes the fixed stack objects, including fixed spill slots.
The fixed stack objects are serialized using a YAML sequence of YAML inline
mappings. Each mapping has the object's ID, type, size, offset, and alignment.
The objects that aren't spill slots also serialize the isImmutable and isAliased
flags.

The fixed stack objects are a part of the machine function's YAML mapping.

Reviewers: Duncan P. N. Exon Smith
llvm-svn: 242045