[LLVM-C] Add binding to look up intrinsic by name

Summary: Add a binding to Function::lookupIntrinsicID so clients don't have to go searching the ID table themselves.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59697

llvm-svn: 356948