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

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

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

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