debuginfo: Basic support for trait objects.
debuginfo: Implement DI for ty_opaque_box.
debuginfo: Support for recursive types.
debuginfo: Added test cases for recursive structs.
debuginfo: Added description of algorithm for handling recursive types.

Also fixed nasty bug caused by calling LLVMDIBuilderCreateStructType() with a null pointer where an empty array was expected (which would trigger an unintelligable assertion somewhere down the line).
debuginfo: Fix style nits for pull request.
debuginfo: Some namespace-related cleanup.
auto merge of #9168 : michaelwoerister/rust/traits, r=jdm

This pull request finally adds support for recursive type definitions and provides a stub implementation for object pointers.