I recommend the LLVM source code. I've worked a bit with it implementing toy compilers and using LLVM as a backend for generating x86/SPARC object code. Just compiling it is a nice exercise if you're new to C++ and CMake (as I was, then).
I would start by reading the LLVM Programmer's Manual [prog man] and the LLVM Coding Standards [code stand] and looking up how the code they reference and the examples are implemented and how they interact with each other. You can browse the code in the LLVM Mirror Git repository [llvm-git] or the cross-referenced Doxygen [llvm-doxy] (has very useful links!). If you're willing to spend the time it might be worth setting up the codebase in your IDE of choice with the proper integrations to be able to browse and cross-reference the source code there.
There is a lot to learn especially from the Important an
Read the rest
d useful LLVM APIs and Picking the Right Data Structure for a Task (and the corresponding code) in the Programmer's Manual. The data structures under llvm/ADT could be especially useful to look at. They are good generic data structures that you may even want to directly reuse in your own project, but they are much easier on the eyes and closer to what you and I would want to see in our own codebases than the source code of the standard library or the Boost data structures.
[prog man]: https://releases.llvm.org/4.0.1/docs/ProgrammersManual.html
[code stand]: https://releases.llvm.org/4.0.1/docs/CodingStandards.html
[llvm-git]: https://github.com/llvm-mirror/llvm
[llvm-doxy]: http://llvm.org/doxygen/modules.html
PS. The Coding Standards will contain a lot of subjective stuff that makes sense inside LLVM for consistency but you don't want to adopt verbatim into an existing (maybe not even a green field) project, like naming variables with Capital letters. You can skim these parts, but it might be useful to know about the existence of these types of issues.