Cool, but why use Clang (and not e.g. GCC 4.7 which is written in C, supports all of Orthodox C++, and is much faster than Clang with very good compilation spee

Cool, but why use Clang (and not e.g. GCC 4.7 which is written in C, supports all of Orthodox C++, and is much faster than Clang with very good compilation speed and optimization capabilities) in the first place?