Add support for printing out the attribute groups.

This emits the attribute groups that are used by the functions. (It currently
doesn't print out return type or parameter attributes within attribute groups.)

Note: The functions still retrieve their attributes from the "old" bitcode
format (using the deprecated 'Raw()' method). This means that string attributes
within an attribute group will not show up during a disassembly. This will be
addressed in a future commit.