Defensively ensure that GetExternalDeclStmt protects itself from nested deserialization

llvm-svn: 297322

clang/lib/Serialization/ASTReader.cpp

@@ -6811,6 +6811,9 @@ Stmt *ASTReader::GetExternalDeclStmt(uint64_t Offset) {
   // Offset here is a global offset across the entire chain.
   RecordLocation Loc = getLocalBitOffset(Offset);
   Loc.F->DeclsCursor.JumpToBit(Loc.Offset);
+  assert(NumCurrentElementsDeserializing == 0 &&
+         "should not be called while already deserializing");
+  Deserializing D(this);
   return ReadStmtFromStream(*Loc.F);
 }