[LLVM-C] Improve Bindings For Aliases

Summary: Add wrappers for a module's alias iterators and a getter and setter for the aliasee value.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits, harlanhaskins

Differential Revision: https://reviews.llvm.org/D46808

llvm-svn: 332826