Switch to using upstream prebuilt binaries for snap package

[WebDrake]

This is a major (major major MAJOR) change in the package design which carries 3 major benefits:

• it GREATLY simplifies the snap package design itself, taking advantage of the existing release packages that are designed for cross-distro, multi-arch support and reducing snap build times from hours to seconds

• it allows us to support ARM64 as an install target

• it precisely matches functionality between upstream releases and the snap package

There are however also some major costs:

• it removes our ability to support i386 install targets, as upstream does not supply i386 builds

• it assumes that the upstream build settings will match the needs of the snap package

If this approach is too heavy-handed then it might be possible to make a compromise such as downloading only prebuilt LLVM and still building the actual compiler and tool executables.

[joseph-wakeling-frequenz]

@kinke @JohanEngelen I'd be interested in your thoughts/preferences on this. I also need to test out the results across different distros somewhat.

[kinke]

As I've mentioned multiple times, I have personally no use for snap packages, and I've never understood why anyone would prefer such a (Canonical-specific/proprietary?) system over a simple download+extract (or upstream's install.sh or the like if that's 'too much work'). Anyway, simply wrapping the official packages into such a package makes probably much more sense than building a near-identical thing manually from scratch.

Wrt. i686, official Ubuntu support was dropped with 19.10 AFAIK, so I doubt anyone will miss it (and cross-compilation from an x64 host is still possible).

[joseph-wakeling-frequenz]

Thanks for the thoughts; I'd forgotten about the 19.10 dropping of i686, so good to have a reminder of why the 32-bit snaps are not needed. My concern was mainly to flag any potential issues that you might anticipate; there are clearly none, so let's push forward with this.

[WebDrake]

OK, this is looking good across multiple distros. Let's take it in: it's far simpler than all the aternatives.