Debug Info: this reverts commit r189600.

We had further discussions on how to retain types, whether to do it in front end
or in DIBuilder. And we agree to do it in DIBuilder so front ends
generating unique identifier do not need to worry about retaining them.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189609 91177308-0d34-0410-b5e6-96231b3b80d8