Add binding for counting argument and find called value of call and invoke...

Add binding for counting argument and find called value of call and invoke instruction from the C API.

Summary: As per title. This remove the need to rely on internal knowledge of call and invoke instruction to find called value and argument count.

Reviewers: bogner, chandlerc, echristo, dblaikie, joker.eph, Wallbraker

Differential Revision: http://reviews.llvm.org/D17054

llvm-svn: 260332