[clang] [analyzer][NFC] Remove outdated FIXME comment (PR #68211)

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https://github.com/gamesh411 approved this pull request.

I can confirm the claims about HeapSpaceRegion being used:
![image](https://github.com/llvm/llvm-project/assets/3802256/682c69f1-7129-4d0f-873f-44aee0fa5f2f)



https://github.com/llvm/llvm-project/pull/68211