This is the final step/commit for implementing exlicit implicit casts. Unlike the 
previous two checkins, which involved lot's of tedious refactoring, this checkin is nice and clean:-)

- Hacked UsualUnaryConversions, UsualArithmeticConversions, and DefaultFunctionArrayConversion
to create the AST node (using a helper function promoteExprToType).
- Added a setType method to Expr.
- Changed Expr::isIntegerConstantExpr to allow for the new node.

llvm-svn: 39866