BitcodeReader: Avoid bad debug info warnings when stripping

[tpr]

Summary

When loading old bitcode where the debug info would need upgrading, but
I am asking to strip debug info, it would give warnings about bad debug
info anyway. This commit stops it doing that.

[aprantl]

I think agree with the idea of the patch. Can you please check that (1) there is an existing test that ensures warning is printed when not explicitly stripping debug info and (2) add a test for this case?

[aprantl]

The debug info "upgrade" mechanism is unintuitive that I'm worried that this could subtly break intended behavior if it isn't tested.

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

Error BitcodeReader::materializeModule() {
   UpgradedIntrinsics.clear();
   // Do the same for remangled intrinsics
   for (auto &I : RemangledIntrinsics) {
     I.first->replaceAllUsesWith(I.second);
     I.first->eraseFromParent();
   }
   RemangledIntrinsics.clear();
 
+  if (StripDebugInfo)
+    llvm::StripDebugInfo(*TheModule);
+  else
     UpgradeDebugInfo(*TheModule);
 
   UpgradeModuleFlags(*TheModule);
 
   UpgradeARCRuntime(*TheModule);
 
   return Error::success();
 }