Make devel/qt5-qdoc use LLVM_DEFAULT instead of hard-coded version
Having the version hard-coded makes it hard to chase LLVM updates;
having a different LLVM version from the rest of the stack -- in particular
from what mesa uses -- means building an extra LLVM just for parsing
documentation from Qt modules.
linimon@ suggests using an easily-spotted variable in PR 239740
but we can go one better and just use the LLVM that's there.
Submitted by: linimon