See commit for details.
So I'm a bit annoyed about this unexpected removal of LLVM from the runtime. It shouldn't compile right now, because the LLVM extension is only available for stable versions of the runtime (so not fdo 21.08beta that the master GNOME sdk depends on). Also, it isn't autodownloaded by Builder, obviously as it's not available for the devel fdo sdk. So I've no idea what's the way to go for nightly builds. I asked about this whole stuff in the release team's channel but I was said that for stable builds there's the llvm sdk extension on flathub stable, but for nightly/development build it was proposed to build LLVM as part of the manifest, which obviously is not a viable solution. I've just asked in the channel if it was possible to get it built on gnome-nightly, or if any other solution was possible. I'm creating this MR now just so that it is known, we'll see what alternative is proposed there tomorrow.