Expose TailCallKind via the LLVM C API
Summary: This exposes CallInst's tail call kind via new LLVMGetTailCallKind and LLVMSetTailCallKind functions. The motivation for this is to be able to see musttail for languages that require mandatory tail calls for correctness. Today only the weaker LLVMSetTail is exposed and there is no way to set GuaranteedTailCallOpt via the C API.
Reviewers: CodaFi, jyknight, deadalnix, rnk
Reviewed By: CodaFi
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D66061