The LLVM logo is a stylized wyvern (a kind of dragon). Dragons have connotations of power, speed and intelligence, and can also be sleek, elegant, and modular (err, maybe not). In addition, there is a series of influential compiler books going back to 1977 which featured dragons on the cover.
Here is the image in a couple of sizes:
Here are several derivative versions of the dragon logo, which may be more suitable for use as icons etc. These were contributed by Teresa Chang:
This dragon image is owned by Apple Inc. and is available for your download and use royalty-free. By downloading this image, Apple grants you, and you accept, a non-exclusive license to use this image. All right, title and interest in the image, including the copyright therein, is retained by Apple.
