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.
Rename isBuiltinCall() to getBuiltinCallee()

This better describes what the function does.

Cleanup only.
Briefly document diag-build.sh

It may be a quick and dirty script but it's still useful to have some
indication as to its purpose.

Text taken straight from Jordan's r158682 commit message.