[ORC] Fix potential stack corruption in Platform::lookupInitSymbols.

We can't exit early when on error here as some threads may still be holding
references to LookupMutex.

Since we don't need high performance in the error case the easy solution is to
drop the early-exit in the error case and wait for all tasks to complete before
returning the error.

Thanks to Jameson Nash for spotting this bug!

Author: lhames

llvm/lib/ExecutionEngine/Orc/Core.cpp

@@ -1530,7 +1530,7 @@ Expected<DenseMap<JITDylib *, SymbolMap>> Platform::lookupInitSymbols(
   }
 
   std::unique_lock<std::mutex> Lock(LookupMutex);
-  CV.wait(Lock, [&] { return Count == 0 || CompoundErr; });
+  CV.wait(Lock, [&] { return Count == 0; });
 
   if (CompoundErr)
     return std::move(CompoundErr);