[lldb] Fix NSURL data formatter truncation issue

Remove hardcoded string prefix length assumption causing issues when
concatenating summary for NSURL in NSURLSummaryProvider. Provider relies
on concatenation of NSStringProvider results for summary, and while the
strings are prefixed with '@' in Objective-C, that is not the case in
Swift causing part of the description to be truncated.

This will be tested in the downstream fork.

Patch by Martin Svensson!