[clang] [Clang][Sema] Suppress -Wconstant-conversion in unselected conditional operands (PR #226775)

[via cfe-commits]

filaka771 wrote:

Following the [feedback on compile-time overhead](https://github.com/llvm/llvm-project/issues/223923#issuecomment-6192146930), I've reworked the implementation to defer condition evaluation until an integer conversion becomes a candidate for `-Wconstant-conversion`. Instead of passing an eagerly computed `IsKnownUnreachable` flag, the analysis records the enclosing conditional operands. When a candidate warning needs a reachability decision, it checks that path and caches the results, so multiple conversions under the same conditional don't repeatedly evaluate its condition.

https://github.com/llvm/llvm-project/pull/226775