Reapply Marek's patch after tagging 1-2-2

Reapply Marek's patch after tagging 1-2-2

2006-11-30  Marek Safar  <marek.safar@gmail.com>

        Correct unary operators implementation (part I)
        Also fixes #80026

        * cfold.cs (Error_CompileTimeOverflow): Made internal

        * const.cs (IConstant): Changed to use reference to constant and
        not constant itself.
        Updated IConstant implementations.

        * constant.cs (CreateConstant): New factory method.
        Updated IConstant implementation.

        * convert.cs (ImplicitStandardConversionExists): Uses compiler Equals.

        * ecore.cs: Updated to use CreateConstantReference.

        * enum.cs: Reflects IConstant changes.

        * expression.cs (Unary): Reimplemented +,-,~ to conform C# standard.

        * literal.cs (NullConstant): Change to be independently usable.

svn path=/trunk/mcs/; revision=68831