[llvm][Bitcode] Add bitcode reader/writer for DSOLocalEquivalent

This is necessary for compilation with [thin]lto.

Differential Revision: https://reviews.llvm.org/D96170

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2888,6 +2888,20 @@ Error BitcodeReader::parseConstants() {
       V = BlockAddress::get(Fn, BB);
       break;
     }
+    case bitc::CST_CODE_DSO_LOCAL_EQUIVALENT: {
+      if (Record.size() < 2)
+        return error("Invalid record");
+      Type *GVTy = getTypeByID(Record[0]);
+      if (!GVTy)
+        return error("Invalid record");
+      GlobalValue *GV = dyn_cast_or_null<GlobalValue>(
+          ValueList.getConstantFwdRef(Record[1], GVTy));
+      if (!GV)
+        return error("Invalid record");
+
+      V = DSOLocalEquivalent::get(GV);
+      break;
+    }
     }
 
     assert(V->getType() == flattenPointerTypes(CurFullTy) &&