Reduce garbage generated in StringBuilder marshaling

Reduce garbage generated in StringBuilder marshaling

When marshaling a StringBuilder In as Ansi (both done by default on Unix), we currently allocate enough native memory to hold the result.  But then we use AnsiCharMarshaler.DoAnsiConversion to allocate a managed byte array and fill it with the converted results, and then we copy those results into the native memory.  We can instead just call String's ConvertToAnsi directly, avoiding the extra managed array allocation and the extra memory copy.