DIBuilder: take an optional StringRef to pass in unique identifier.

createClassType, createStructType, createUnionType, createEnumerationType,
and createForwardDecl will take an optional StringRef to pass in
the unique identifier.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189410 91177308-0d34-0410-b5e6-96231b3b80d8