LLVM is participating in GSoC again this year:
I have checked, and the clang project is officially considered to be part of the umbrella llvm project as far as GSoC is concerned. This means that we are permitted to support clang-related projects as well as llvm-core projects! If you are interested in participating, please send your proposal to the llvmdev mailing list, where a majority of the GSoC discussion is taking place.
If you have specific interests and would like some ideas, please feel free to ask here on cfe-dev. I hope we get a couple of interested students working on clang this summer, GSoC is a great program!