[mlir] ExecutionEngine: fix assertion on the error path

[ftynse]

Summary

MLIR ExecutionEngine and derived tools (e.g., mlir-cpu-runner) would trigger an
assertion inside ORC JIT while ExecutionEngine is being destructed after a
failed linking due to a missing function definition. The reason for this is the
JIT lookup that may return an Error referring to strings stored internally by
the JIT. If the Error outlives the ExecutionEngine, it would want have a
dangling reference, which is currently caught by an assertion inside JIT thanks
to hand-rolled reference counting. Rewrap the error message into a string
before returning.

[ftynse]

drop debug

mlir/lib/ExecutionEngine/ExecutionEngine.cpp

mainJD.addGenerator(
       cantFail(DynamicLibrarySearchGenerator::GetForCurrentProcess(
           dataLayout.getGlobalPrefix())));
 
   return std::move(engine);
 }
 
 Expected<void (*)(void **)> ExecutionEngine::lookup(StringRef name) const {
   auto expectedSymbol = jit->lookup(makePackedFunctionName(name));
-  if (!expectedSymbol)
-    return expectedSymbol.takeError();
+
+  // JIT lookup may return an Error referring to strings stored internally by
+  // the JIT. If the Error outlives the ExecutionEngine, it would want have a
+  // dangling reference, which is currently caught by an assertion inside JIT
+  // thanks to hand-rolled reference counting. Rewrap the error message into a
+  // string before returning. Alternatively, ORC JIT should consider copying
+  // the string into the error message.
+  if (!expectedSymbol) {
+    std::string errorMessage;
+    llvm::raw_string_ostream os(errorMessage);
+    llvm::handleAllErrors(expectedSymbol.takeError(),
+                          [&os](llvm::ErrorInfoBase &ei) { ei.log(os); });
+    return make_string_error(os.str());
+  }
+
   auto rawFPtr = expectedSymbol->getAddress();
   auto fptr = reinterpret_cast<void (*)(void **)>(rawFPtr);
   if (!fptr)
     return make_string_error("looked up function is null");
   return fptr;
 }
 
 Error ExecutionEngine::invoke(StringRef name, MutableArrayRef<void *> args) {