As of LLVM release 3.0, "llvm-gcc is no longer supported, and not included in the release. We recommend switching to Clang or DragonEgg" Additionally the package has multiple bugs opened (see dependency tree). Now that 3.0 packages are in tree, you can switch to its replacement sys-devel/dragonegg or llvm compiler sys-devel/clang sys-devel/llvm-gcc will be removed in 30 days (2012/01/02)
llvm-gcc just removed from tree, have fun with clang and/or dragonegg