Introduce LLVMDIBuilderRef

Summary:
This patch adds a definition of `LLVMDIBuilderRef` that represents an `llvm::DIBuilder`.

Authored by Harlan Haskins

Reviewers: deadalnix, aprantl, probinson, dblaikie, echristo, whitequark

Reviewed By: deadalnix, whitequark

Subscribers: CodaFi, loladiro

Differential Revision: https://reviews.llvm.org/D32122

llvm-svn: 300843