Add C binding for AllocaInst::getAllocatedType

Summary:
Comes with an awesome test.

Depends on D16912

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

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16942

llvm-svn: 260313