More precisely, it is licensed under GPLv3 + runtime exception with the result being pretty much LGPL. In the case of libgcc, for 99.9% of the users there is no difference between this license and BSD.
What I am trying to say is that to me the user of libgcc or the LLVM version there is not difference between BSD and LGPL conditions.
The LLVM project isn't controlled by a single entity, but it does get a lot of code from "commercial" users, so those users chose a license that works well for them. (It ensures that any future contributions will be usable under an acceptable license.) Admittedly the LGPL would also be fine, but ... that's not what they picked.