[clang] [clang][dataflow] Discard unneeded `ExprToLoc` and `ExprToVal` entries. (PR #72850)

[Yitzhak Mandelbaum via cfe-commits]

https://github.com/ymand requested changes to this pull request.

Overall seemed good (mostly just piping), but I think we need more explanation (on the review thread and somewhere appropriate in the code) of what exactly determines whether an expression is "needed".

I was wondering, when reading the code, why *any* expression is needed once we've finished processing the block. My recollection is that ternary expressions are responsible for this, but in that case shouldn't we be looking at them directly? If not, it seems worth explaining.

https://github.com/llvm/llvm-project/pull/72850