[clang] [LifetimeSafety] Add missing origins stats for lifetime analysis (PR #166568)

[Utkarsh Saxena via cfe-commits]

================
@@ -101,6 +105,9 @@ class AnalysisBasedWarnings {
   /// a single function.
   unsigned MaxUninitAnalysisBlockVisitsPerFunction;
 
+  /// Map from expressions missing origin in OriginManager to their counts.
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usx95 wrote:

Let's keep this documentation generic and not specific to missing origins.
Suggestion:
```
/// Statistics collected during lifetime safety analysis.
/// These are accumulated across all analyzed functions and printed
/// when -print-stats is enabled.
```

https://github.com/llvm/llvm-project/pull/166568