Fix compiler error in Attributes.cpp

Fix compiler error in Attributes.cpp
```
Compiling Attributes.cpp ...
../../../Attributes.cpp: In member function 'std::__1::pair<unsigned int, llvm::Optional<unsigned int> > llvm::AttributeSet::getAllocSizeArgs() const':
../../../Attributes.cpp:542:69: error: operands to ?: have different types 'std::__1::pair<unsigned int, llvm::Optional<unsigned int> >' and 'std::__1::pair<int, int>'
   return SetNode ? SetNode->getAllocSizeArgs() : std::make_pair(0, 0);
                                                                     ^
../../../Attributes.cpp:543:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
```

Differential Revision: https://reviews.llvm.org/D31981

llvm-svn: 300143