fix(llvm): +visionary has a limited supported version range

packages/llvm/package.py

@@ -396,6 +396,9 @@ class Llvm(CMakePackage, CudaPackage):
             description='Force linking of all files into view, including '
                         'known conflicts (e.g. libgomp).')
 
+    conflicts("@:8", when="+visionary")
+    conflicts("@13:", when="+visionary")
+
     patch('llvm9-0001-Tooling-Fully-qualify-template-parameters-of-nested-.patch', when='@9.0.0:12.0.999 +visionary', level=2)
 
     # 0001-Tooling-Fully-qualify-template-parameters-of-nested-.patch from above