[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#123499)

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 Init to be nonnull.  Note that hasInit returns true
only if Init is nonnull among other conditions.

Author: kazutakahirata

clang/lib/AST/Decl.cpp

@@ -2399,7 +2399,7 @@ Expr *VarDecl::getInit() {
   if (!hasInit())
     return nullptr;
 
-  if (auto *S = Init.dyn_cast<Stmt *>())
+  if (auto *S = dyn_cast<Stmt *>(Init))
     return cast<Expr>(S);
 
   auto *Eval = getEvaluatedStmt();