[clang] [StaticAnalyzer] Migrate away from PointerUnion::{is,get} (NFC) (PR #118421)

[Kazu Hirata via cfe-commits]

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@@ -1068,10 +1068,10 @@ const VarRegion *MemRegionManager::getVarRegion(const VarDecl *D,
     llvm::PointerUnion<const StackFrameContext *, const VarRegion *> V =
       getStackOrCaptureRegionForDeclContext(LC, DC, D);
 
-    if (V.is<const VarRegion*>())
-      return V.get<const VarRegion*>();
+    if (isa<const VarRegion *>(V))
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kazutakahirata wrote:

Switched to `dyn_cast_if_present` because `V` could be null.

https://github.com/llvm/llvm-project/pull/118421