Handle LLVM_DEFAULT downgrade with DEFAULT_VERSIONS+=llvm=10

[jbeich]

Summary

Fixes bug 245178 and similar.

Test Plan

$ env -i __MAKE_CONF= DEFAULT_VERSIONS=llvm=60 make -V LLVM_DEFAULT -C graphics/mesa-dri
60
$ env -i __MAKE_CONF= DEFAULT_VERSIONS=llvm=10 make -V LLVM_DEFAULT -C graphics/mesa-dri
80

[jbeich]

• Fix typo when propagating the fix to non-mesa ports (& got expanded by sed(1))

[zeising]

Nice catch, thanks!