Revision 535352

MFH: r534936

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)
Approved by:	ports-secteam (joneum)
Differential Revision:	https://reviews.freebsd.org/D24222