[Distributed] More IRGenMangler fixes for distributed thunks

We also need to mangle the dispatch thunks to distributed thunks
uniquely.

Author: ktoso

lib/IRGen/IRGenMangler.h

@@ -51,6 +51,8 @@ class IRGenMangler : public Mangle::ASTMangler {
     beginMangling();
     appendEntity(func);
     appendOperator("Tj");
+    if (func->isDistributedThunk())
+      appendSymbolKind(SymbolKind::DistributedThunk);
     return finalize();
   }