More changes related to implementing ImplicitCastExpr.

- Fixed a recent regression discovered by Keith Bauer (thanks!). 
The fix involved adding (back) two arguments to UsualArithmeticConversions.
Without the reference arguments, no unary conversions were being passed back
to the caller. This had the effect of turning off the UsualUnaryConversions.
- Refactored CheckAssignmentConstraints into 3 functions. CheckAssignmentConstraints,
CheckSingleAssignmentConstraints, and CheckCompoundAssignmentConstraints. 
- Changed the argument type of DefaultFunctionArrayConversion from QualType->Expr*&.
- Removed a bunch of casts in routines I was working on (cleanup).
- Fixed the visitor for ImplicitCastExpr (oops).

llvm-svn: 39840