Update VisualStudio.gitignore to add a safer rule for rdl backups (#3047)

VisualStudio.gitignore

@@ -262,7 +262,9 @@ ServiceFabricBackup/
 *.bim.layout
 *.bim_*.settings
 *.rptproj.rsuser
-*- Backup*.rdl
+*- [Bb]ackup.rdl
+*- [Bb]ackup ([0-9]).rdl
+*- [Bb]ackup ([0-9][0-9]).rdl
 
 # Microsoft Fakes
 FakesAssemblies/