Thread Safety Analysis: some minor cleanups to the latest thread safety changes. No functional changes intended.

* Adds an iterator_range interface to CallExpr to get the arguments
* Modifies SExpr such that it must be allocated in the Arena, and cannot be deleted
* Minor const-correctness and nullptr updates
* Adds some operator!= implementations to complement operator==
* Removes unused functionality


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205915 91177308-0d34-0410-b5e6-96231b3b80d8