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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@290987 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm