[clang] [clang][dataflow] Allow `DataflowAnalysisContext` to use a non-owned `Solver`. (PR #91316)

[Mehdi Amini via cfe-commits]

joker-eph wrote:

CI failure wasn't unrelated actually: the CI is consistently broken after this patch (you have got an email about it?), I'll revert for now.

https://github.com/llvm/llvm-project/pull/91316