Added smart pointer class "IntrusiveSPtr" that handles reference

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@42260 91177308-0d34-0410-b5e6-96231b3b80d8