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

[Benedek Kaibas via cfe-commits]

benedekaibas wrote:

Thank you for the review @Xazax-hun and @steakhal 

I have fixed the changes you have suggested:

- Removed comments
- Created the static helper function `getContainerRegion()`
- Applied const to the `ContainerRegion`
- Rewrote `isSymbolTracked()` function to a simpler more readable format

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