MIR Serialization: print and parse machine function names.

This commit introduces a serializable structure called
'llvm::yaml::MachineFunction' that stores the machine
function's name. This structure will mirror the machine 
function's state in the future.

This commit prints machine functions as YAML documents
containing a YAML mapping that stores the state of a machine
function. This commit also parses the YAML documents
that contain the machine functions.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D9841

llvm-svn: 238519