[clang] [analyzer] Remove barely used class 'KnownSVal' (NFC) (PR #86953)

[via cfe-commits]

================
@@ -1238,12 +1238,12 @@ class StoreSiteFinder final : public TrackingBugReporterVisitor {
   ///        changes to its value in a nested stackframe could be pruned, and
   ///        this visitor can prevent that without polluting the bugpath too
   ///        much.
-  StoreSiteFinder(bugreporter::TrackerRef ParentTracker, KnownSVal V,
+  StoreSiteFinder(bugreporter::TrackerRef ParentTracker, SVal V,
                   const MemRegion *R, TrackingOptions Options,
                   const StackFrameContext *OriginSFC = nullptr)
       : TrackingBugReporterVisitor(ParentTracker), R(R), V(V), Options(Options),
         OriginSFC(OriginSFC) {
-    assert(R);
+    assert(!V.isUnknown() && R);
----------------
NagyDonat wrote:

Actually, I realized that it's better to avoid asserting that `V` is not unknown. This is a precondition that _happens to be satisfied_ because this class is constructed in one single location and that is preceded by a `!V.isUnknown()` check -- but I'm fairly certain that the actual implementation of `StoreSiteFinder` can find a site where `UnknownVal` was stored in a region. 

https://github.com/llvm/llvm-project/pull/86953