Harmless debug assert hit in Uri's DebugAssertInCtor

[MihaZupan]

// Uri.CreateThisFromUri fires Debug.Assert: (_flags & Flags.Debug_LeftConstructor) == 0
// when combining a UNC file URI base with a "file:" relative URI.
//
// Source: System.Uri.DebugAssertInCtor() at Uri.cs:205
//         System.Uri.CreateThisFromUri() at UriExt.cs:891

var baseUri = new Uri(@"\\aa", UriKind.Absolute);   // parses as file://aa/
var relUri = new Uri("file:", UriKind.Relative);
_ = new Uri(baseUri, relUri);                        // Debug.Assert failure

The assert violation is in CreateThisFromUri — it expects the constructor flag Debug_LeftConstructor to not be set, but the re-entrant URI construction from a UNC base + file: relative violates this invariant.

This is from the extra DebugAssertInCtor I added to DebugSetLeftCtor in #123932. We should either remove the assert or avoid copying the Debug_LeftConstructor flag in CreateThisFromUri.

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