Optimize StringWriter scenarios involving SetWriteSizeHint(): res…

…ize the

destination to capacity and set up the buffer right at the end of
`SetWriteSizeHintImpl()`, instead of waiting for e.g. `Push()` falling to
`PushSlow()`, synchronizing the buffer, figuring out that the current capacity
will be sufficient, and finally doing the same.

This speeds up some simple usages by about 10%.

PiperOrigin-RevId: 566338632

riegeli/bytes/string_writer.cc

@@ -108,6 +108,7 @@ void StringWriterBase::SetWriteSizeHintImpl(
     std::move(secondary_buffer_).AppendTo(dest);
     secondary_buffer_.Clear();
   }
+  GrowDestToCapacityAndMakeBuffer(dest, IntCast<size_t>(start_pos()));
 }
 
 bool StringWriterBase::PushSlow(size_t min_length, size_t recommended_length) {