Add support for do/while loops

This commit adds support for a standard do <statement> while <expr>;
form of statement.  While loops were already being supported; supporting
a do/while loop mostly involves reusing code from while loops but putting
the conditional checking last.

libsolidity/ast/ASTPrinter.cpp

@@ -208,7 +208,7 @@ bool ASTPrinter::visit(IfStatement const& _node)
 
 bool ASTPrinter::visit(WhileStatement const& _node)
 {
-	writeLine("WhileStatement");
+	writeLine(_node.isDoWhile() ? "DoWhileStatement" : "WhileStatement");
 	printSourcePart(_node);
 	return goDeeper();
 }