MIR Serialization: Create dummy functions when the MIR file doesn't have LLVM IR.

This commit creates a dummy LLVM IR function with one basic block and an unreachable
instruction for each parsed machine function when the MIR file doesn't have LLVM IR.
This change is required as the machine function analysis pass creates machine
functions only for the functions that are defined in the current LLVM module.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10135

llvm-svn: 239778