[clang] [analyzer][NFC] Document and centralize the CallEvent argument/parameter index mapping (PR #221252)

[Benedek Kaibas via cfe-commits]

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@@ -1073,6 +1073,10 @@ class ParamVarRegion : public VarRegion {
   friend class MemRegionManager;
 
   const Expr *OriginExpr;
+
+  /// Index of teh declared parameter of the callee that this region stands
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benedekaibas wrote:

Applied changes here: [25f5339](https://github.com/llvm/llvm-project/pull/221252/commits/25f5339a6ef0e59ac4e1e478215ac07eadcd46a4)

https://github.com/llvm/llvm-project/pull/221252