[LLVM-C] Add bindings to create enumerators

Summary: The C API don't have the bindings to create enumerators, needed to create an enumeration.

Reviewers: whitequark, CodaFi, harlanhaskins, deadalnix

Reviewed By: whitequark, CodaFi, harlanhaskins

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58323

llvm-svn: 354237