C++ Interop: Don't crash when trying to call a C++ function with undeduced return type (#6363)

This crashed on trying to build a C++ thunk because the error wasn't
propagated.

Part of #5436.

toolchain/check/cpp/import.cpp

@@ -1546,6 +1546,9 @@ static auto GetReturnPattern(Context& context, SemIR::LocId loc_id,
     // void.
     return SemIR::InstId::None;
   }
+  if (type_inst_id == SemIR::ErrorInst::TypeInstId) {
+    return SemIR::ErrorInst::InstId;
+  }
   auto pattern_type_id = GetPatternType(context, type_id);
   clang::SourceLocation return_type_loc =
       clang_decl->getReturnTypeSourceRange().getBegin();