[clang] [clang][dataflow] Fix inaccuracies in `buildStmtToBasicBlockMap()`. (PR #82496)

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martinboehme wrote:

> This is horrifying. It makes me think that we might want to revamp the CfgBlock interface at some point.

Yes, there a quite a few surprises in the structure of the CFG that I've only discovered gradually over time. I'd also like to eliminate the sharp edges in the API, though I suspect that existing clients rely on the idiosyncrasies of the current API -- so we might have to introduce a second, parallel API for a while so we can migrate usage over gradually.

https://github.com/llvm/llvm-project/pull/82496