Added smart pointer class "IntrusiveSPtr" that handles reference

counted objects that maintain their own internal reference count.
This smart pointer implementation is compatible with LLVM-style
down-casting (see in llvm: include/llvm/Support/Casting.h).

Implemented "RefCounted", a base class that objects that wish to be
managed using IntrusiveSPtrs can subclass.

Reference counted objects are being targeted for use in path-sensitive
dataflow analyses where managing many live objects becomes difficult.

llvm-svn: 42260