Refactoring the security checker a little bit so that each CallExpr check...

Refactoring the security checker a little bit so that each CallExpr check doesn't get called for each CallExpr. Instead it does a switch and only runs the check for the proper identifier. Slight speed improvement (probably significant on very large ASTs), and should make it easier and more clear to add more checks for other CallExpr's later.

llvm-svn: 128785