Tidy up CGCXXABI creation

'create' functions conventionally return a pointer, not a reference.

Also use an OwningPtr to get replace the delete of a reference member.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198126 91177308-0d34-0410-b5e6-96231b3b80d8