Thread Safety Analysis: some minor cleanups to the latest thread safety...

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

llvm-svn: 205915