Add document to index.rst

Author: andykaylor

clang/docs/index.rst

@@ -121,7 +121,7 @@ Design Documents
    ControlFlowIntegrityDesign
    HardwareAssistedAddressSanitizerDesign.rst
    ConstantInterpreter
-
+   ClangIRCodeDuplication
 
 Indices and tables
 ==================