Restore "Obtained from string" source name.

This causes a fair number of diffs when importing into Google. If a way of setting the source name for string input streams was provided, then we could have the default be the empty string.

Signed-off-by: Jerry Berg <107155935+googleberg@users.noreply.github.com>

runtime/Go/antlr/v4/input_stream.go

@@ -148,7 +148,7 @@ func (is *InputStream) GetTextFromInterval(i Interval) string {
 }
 
 func (*InputStream) GetSourceName() string {
-	return ""
+	return "Obtained from string"
 }
 
 // String returns the entire input stream as a string