Expose IRBuilder::CreateAtomicCmpXchg as LLVMBuildAtomicCmpXchg in the C API.

Summary: Also expose getters and setters in the C API, so that the change can be tested.

Reviewers: nhaehnle, axw, joker.eph

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18260

From: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
llvm-svn: 263886