llvm-c: use typedef for function pointers

This makes it consistent with other function pointers used in llvm-c

Differential Revision: http://llvm-reviews.chandlerc.com/D1712



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191693 91177308-0d34-0410-b5e6-96231b3b80d8