[clang] [StaticAnalyzer] Fix InnerPointerBRVisitor to avoid repeated map scans (PR #185796)

[Balázs Benics via cfe-commits]

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@@ -59,9 +59,19 @@ class InnerPointerChecker
 public:
   class InnerPointerBRVisitor : public BugReporterVisitor {
     SymbolRef PtrToBuf;
+    const MemRegion *ContainerRegion;
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steakhal wrote:

I wish this was const.

```suggestion
    const MemRegion *const ContainerRegion;
```

https://github.com/llvm/llvm-project/pull/185796