[llvm-jitlink] Shut down the session on an error return path.

Ensures cleanup of task dispatcher threads. This may address some of the
nondeterministic failures seen in llvm-jitlink regression tests recently.

Author: lhames

llvm/tools/llvm-jitlink/llvm-jitlink.cpp

@@ -2562,6 +2562,7 @@ int main(int argc, char *argv[]) {
     if (Timers)
       Timers->JITLinkTG.printAll(errs());
     reportLLVMJITLinkError(EntryPoint.takeError());
+    ExitOnErr(S->ES.endSession());
     exit(1);
   }