This has no relation to Clang's permissive license. The problem here is generally that nobody has implemented the necessary support, not that they have implemented support in their private forks but did not bother to upstream it. A GPL-style license doesn't magically force contributors to start working on a particular feature.

Google at least is one of the largest contributors to the LLVM project -- it's just that their contributions don't tend to focus on Clang frontend work.