you only need to worry about GPLv3 if you are modifying gcc in source and building it and distributing that. Just running gcc does not create a GPLv3 infection. And glibc et al are library licensed so they don't infect what you build either, most especially if you are not modifying its source and rebuilding it.
the context here doesn't actually specify whether we are talking about companies using llvm sources to create proprietary compilers (or maybe integrated with a proprietary IDE) or using llvm to quickly bootstrap and craft a compiler for a new processor, new language, etc., where they will distribute the source to the compiler anyway
but such a compiler or IDE would not GPLv3 infect it's users' target sources and binaries.