MIR Parser: Report an error when a machine function doesn't have a corresponding function.

This commit reports an error when a machine function from a MIR file that contains
LLVM IR can't find a function with the same name in the loaded LLVM IR module.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10468

llvm-svn: 239831