Improvements to IdempotentOperationChecker and its use of PseudoConstantAnalysis

- Added wasReferenced function to PseudoConstantAnalysis to determine if a variable was ever referenced in a function (outside of a self-assignment)
- BlockDeclRefExpr referenced variables are now explicitly added to the non-constant list
- Remove unnecessary ignore of implicit casts
- Generalized parameter self-assign detection to detect deliberate self-assigns of variables to avoid unused variable warnings
- Updated test cases with deliberate self-assignments
- Fixed bug with C++ references and pseudoconstants
- Added test case for C++ references and pseudoconstants

llvm-svn: 111965