Debug Info: Move a helper function getTypeIdentifier from DIBuilder to be part
of DIType.
    
Implement DIType::generateRef to return a type reference. This function will be
used in setContaintingType and in DIBuilder to generete the type reference.
    
No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190188 91177308-0d34-0410-b5e6-96231b3b80d8