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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66061

llvm-svn: 368945