"primarily", because the LLVM / clang project had its origins in academia (before the project and its team got acqui-hired by Apple), and because the project is open source, and thus still gets some amount of community contributions (probably not a huge proportion of the overall effort, though).

It's UIUC's license, which looks similar to BSD to me. They also have some MIT licensed code. http://llvm.org/docs/DeveloperPolicy.html#license

Essentially, yes. It's NCSA-OSL which is based on MIT, which is based on BSD: http://en.wikipedia.org/wiki/Clang

Many projects use Clang/LLVM for their OpenCL implementations.