On amd64, sys-devel/llvm-3.7.1-r2 cannot be installed because of dev-libs/libclc-0.0.1_pre20141027, which requires <sys-devel/clang-3.6 && <sys-devel/llvm-3.6. Version 0.2.0 (any one present in portage) requires >=sys-devel/llvm-3.7 and doesn't block an update. Please, stabilize any of 0.2 version of libclc
There is already a stabilization bug for all X.org packages *** This bug has been marked as a duplicate of bug 579266 ***