Bump LLVM requirement to 10.0

LLVM 10.0 was released in March 2020 so we probably ought to make it the blessed compiler for %8.12.1.

changed milestone to %9.0.1

added LLVM backend Ttask labels

mentioned in merge request ci-images!44 (merged)

This is made slightly problematic by the fact that there are no Debian 9-compatible binary distributions for LLVM 10. This will force quite a few other changes in ghc/ci-images>.

mentioned in issue #15689

I see, just for reference: Debian Experimental already has llvm-10, as does the recent release of Fedora 32.

http://apt.llvm.org/buster/llvm-toolchain-buster-10/main should have llvm-10, but only for Intel archs.

mentioned in commit f468dded

mentioned in merge request !3423 (closed)

mentioned in commit 407c95e6

mentioned in commit b7d5a5f6

mentioned in commit 39a725fe

closed via commit 89f4d8e9

mentioned in commit 89f4d8e9

Ben, how long do you plan to stay on Fedora 27 btw?

I've been meaning to add a Fedora 32 job and eventually retire old Fedora 27 job.