As of r228670, add_llvm_library() supports the ADDITIONAL_HEADER_DIRS argument. This patch uses this argument on every occurrence of add_llvm_library() in llvm. This should greatly increase the number of header files in your IDE project and make source code browsing generally more pleasant.
I did measure a project load time increase of about 10% on Windows with MSVC 2013. I think this is acceptable for MSVC, but would like some feedback from Xcode people to see if the situation is worse there.