[LLVM-C] Add DIBuilder bindings to create import declarations

Summary: Add bindings to create import declarations for modules, functions, types, and other entities.  This wraps the conveniences available in the existing DIBuilder API, but these seem C++-specific.

Reviewers: whitequark, harlanhaskins, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46167

llvm-svn: 331123