Make 'fixed variable' handling standards compliant. Fix #70807, #72729.
* ecore.cs (IVariable.VerifyFixed): Remove 'is_expression' parameter.
(FieldExpr.VerifyFixed): Ensure that the field is part of a fixed
variable of struct type.
* expression.cs (Unary.ResolveOperator): Update to change.
(Indirection.VerifyFixed): Likewise.
(LocalVariableReference.VerifyFixed): A local variable is always fixed.
(ParameterReference.VerifyFixed): Value parameters are fixed.
(This.VerifyFixed): Treat 'this' as a value parameter.
* statement.cs (LocalInfo.IsFixed): Remove.
In tests:
* test-423.cs: New test based on #70807.
In errors:
* cs0212-2.cs: New test from #72729.
svn path=/trunk/mcs/; revision=46942
