Bring Native Clang Support to Gentoo

Clang is a C/C++/Obj-C compiler spawned from the LLVM project, famous for its modular design, rich diagnostic messages, non-intrusive license, etc. Some Unix flavors, including FreeBSD and OS X, have already adopted a purely clang/LLVM based toolchain. It would be interesting and beneficial to have clang as a first-class compiler, in place of GCC, on Gentoo as well. This task involves bootstrapping a freestanding clang along with its supporting libraries, and deploy them on Gentoo.