C#: Visit prefix/markers before VisitExpression/VisitStatement, matching Java

Reorder all visitor methods in JavaVisitor.cs and CSharpVisitor.cs so
that WithPrefix and WithMarkers are called before the
VisitExpression/VisitStatement dispatch, consistent with the Java
JavaVisitor.java reference. Also fix FieldAccess and PointerFieldAccess
to use Expression→Statement ordering matching the Java convention.

Author: knutwannheden