Don't make unqualified calls to functions that could well be found via

ADL as reasonable extension points.

All of this would be cleaner if this code followed the more usual LLVM
convention of not having deeply nested namespaces inside of .cpp files
and instead having a `using namespace ...;` at the top. Then the static
function would be in the global namespace and easily referred to as
`::join`. Instead we have to write a fairly contrived qualified name.
I figure the authors can clean this up with a less ambiguous name, using
the newly provided LLVM `join` function, or any other solution, but this
at least fixes the build.

llvm-svn: 298434