Move interleave(...) to the llvm namespace

This simplifies fixing the master-next build. Upstream LLVM already
has a copy of this function, so on master-next we only need to delete
the Swift copy, reducing the potential for merge conflicts.

lib/FrontendTool/FrontendTool.cpp

@@ -1973,7 +1973,7 @@ static void printTargetInfo(const CompilerInvocation &invocation,
 
   auto outputPaths = [&](StringRef name, const std::vector<std::string> &paths){
     out << "    \"" << name << "\": [\n";
-    interleave(paths, [&out](const std::string &path) {
+    llvm::interleave(paths, [&out](const std::string &path) {
       out << "      \"";
       out.write_escaped(path);
       out << "\"";