Cmm files without return type information

[kavon]

As the TODO(kavon) in compiler/cmm/CmmParse.y says, we haven't yet updated the Cmm grammar to allow for return type information to be specified.

This will become problematic for any handwritten Cmm that returns floats or vectors [1]. I doubt we have any right now, but this needs fixing.

[1] This is because the return type for the LLVM version of the function will be incorrect.