Adapt LLVM_DEFAULT downgrades to the new version scheme in devel/llvm10

$ DEFAULT_VERSIONS=llvm=90 make -V LLVM_DEFAULT -C graphics/mesa-dri
80
$ DEFAULT_VERSIONS=llvm=10 make -V LLVM_DEFAULT -C graphics/mesa-dri
10

PR: 245178
Reported by: mi
Approved by: x11 (zeising), maintainer timeout (2 months)
MFH: 2020Q2 (for llvm10 users)
Differential Revision: https://reviews.freebsd.org/D24222