[LLVM-C] Add bindings to insert basic blocks

Summary:
Now that we can create standalone basic blocks, it's useful to be able to append them.  Add bindings to

- Insert a basic block after the current insertion block
- Append a basic block to the end of a function's list of basic blocks

Reviewers: whitequark, deadalnix, harlanhaskins

Reviewed By: whitequark, harlanhaskins

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59658

llvm-svn: 357812