[AST] Migrate away from PointerUnion::dyn_cast (NFC)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect Source to be nonnull.

Author: kazutakahirata

clang/lib/AST/ByteCode/EvaluationResult.cpp

@@ -160,9 +160,9 @@ bool EvaluationResult::checkFullyInitialized(InterpState &S,
     return true;
 
   SourceLocation InitLoc;
-  if (const auto *D = Source.dyn_cast<const Decl *>())
+  if (const auto *D = dyn_cast<const Decl *>(Source))
     InitLoc = cast<VarDecl>(D)->getAnyInitializer()->getExprLoc();
-  else if (const auto *E = Source.dyn_cast<const Expr *>())
+  else if (const auto *E = dyn_cast<const Expr *>(Source))
     InitLoc = E->getExprLoc();
 
   if (const Record *R = Ptr.getRecord())