[clang] [analyzer][NFC] Cleanup BranchNodeBuilder (PR #117898)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

I didn't run an analysis yet, because I was convinced about the correctness of this change based on an understanding of the source code (and the fact that the lit tests pass). However, I started a test run now to double-check this. I'll merge this commit if it reveals no discrepancies.

https://github.com/llvm/llvm-project/pull/117898