Add AdvanceAggregatesCodegenEnroll mock function

This function is used in codegen unittests.
In this commit, we also address a minor cpplint error in GenerateSlotGetAttr.

src/backend/codegen/slot_getattr_codegen.cc

@@ -662,7 +662,8 @@ bool SlotGetAttrCodegen::GenerateSlotGetAttr(
   // Note: We collect error code information, based on the block from which we
   // fall back, and log it for debugging purposes.
   irb->SetInsertPoint(fallback_block);
-  llvm::PHINode* llvm_error = irb->CreatePHI(codegen_utils->GetType<char *>(), 2);
+  llvm::PHINode* llvm_error = irb->
+      CreatePHI(codegen_utils->GetType<char *>(), 2);
   llvm_error->addIncoming(codegen_utils->GetConstant("slot check failed"),
       slot_check_block);
   llvm_error->addIncoming(codegen_utils->GetConstant("tuple check failed"),

src/test/unit/mock/gpcodegen_mock.c

@@ -110,3 +110,13 @@ ExecEvalExprCodegenEnroll(ExecEvalExprFn regular_func_ptr,
    return NULL;
 }
 
+// Enroll and returns the pointer to AdvanceAggregateGenerator
+void*
+AdvanceAggregatesCodegenEnroll(AdvanceAggregatesFn regular_func_ptr,
+		AdvanceAggregatesFn* ptr_to_regular_func_ptr,
+		struct AggState *aggstate) {
+	*ptr_to_regular_func_ptr = regular_func_ptr;
+	elog(ERROR, "mock implementation of AdvanceAggregatesCodegenEnroll called");
+	return NULL;
+}
+