ADT: IntrusiveRefCntPtr: Broaden the definition of correct usage of RefCountedBase

This roughly matches the semantics of std::enable_shared_from_this - that it
does not dictate the ownership model of all users, but constrains those users
taking advantage of the intrusive nature to do so only when there's a guarantee
that that's the ownership model being used for the object being passed.

Reviewers: jlebar

Differential Revision: https://reviews.llvm.org/D28245

llvm-svn: 290987