This is the final step/commit for implementing exlicit implicit casts. Unlike the

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@39866 91177308-0d34-0410-b5e6-96231b3b80d8