devel/llvm: allow versions other than LLVM_DEFAULT

Allow the LLVM port to depend on and link to to be something other
than LLVM_DEFAULT. This port can now be build with LLVM_SUFFIX set
to other values including non-numeric values such as -devel, -cheri,
or -morello. To accomodate this, change how the version is
set so we use the major version rather than the suffix. This requires
a PORTEPOCH bump.