[MLIR] Fix gcc-5 build failure cause by D77528

[bondhugula]

Summary

Fix gcc-5 build failure cause by D77528

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

 };
 
 struct AllocOpLowering : public AllocLikeOpLowering<AllocOp> {
   explicit AllocOpLowering(LLVMTypeConverter &converter,
                            bool useAlignedAlloc = false)
       : AllocLikeOpLowering<AllocOp>(converter, useAlignedAlloc) {}
 };
 
-struct AllocaOpLowering : public AllocLikeOpLowering<AllocaOp> {
-  using Base::Base;
-};
+using AllocaOpLowering = AllocLikeOpLowering<AllocaOp>;
 
 // A CallOp automatically promotes MemRefType to a sequence of alloca/store and
 // passes the pointer to the MemRef across function boundaries.
 template <typename CallOpType>
 struct CallOpInterfaceLowering : public ConvertOpToLLVMPattern<CallOpType> {
   using ConvertOpToLLVMPattern<CallOpType>::ConvertOpToLLVMPattern;
   using Super = CallOpInterfaceLowering<CallOpType>;
   using Base = ConvertOpToLLVMPattern<CallOpType>;