[LLVM-C][OCaml] Add C and OCaml APIs for llvm::StructType::isLiteral

Summary:
This patch adds LLVMIsLiteralStruct to the C API to expose
StructType::isLiteral. This is then used to implement the analogous
addition to the OCaml API.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52209

llvm-svn: 342435