[clang] [WIP][analyzer] Refactor `ExplodedGraph::trim()` (PR #139939)

[Balazs Benics via cfe-commits]

steakhal wrote:

> I started a measurement on open source projects to see the effect of this change on the (total) analysis runtime. I don't expect much but if this turns out to be non-negligible, then I will prioritize this direction.

RT is not a concern to me in this case.

https://github.com/llvm/llvm-project/pull/139939