Nether llvm nor clang are GPL, dual licensed or otherwise. They're Apache 2.0[1] (or formerly a quasi-MIT/X11 license[2]), as is swift[3].
The binaries of the compilers they release with the platform are also built with internal forks that have some differences with the public llvm/clang/swift trees and that wouldn't be possible with gpl.
[1] https://github.com/llvm/llvm-project/blob/main/llvm/LICENSE....
[2] https://github.com/llvm/llvm-project/blob/7555c589af006c9c4d...