[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#124446)

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 Pattern to be nonnull.

Author: kazutakahirata

clang/lib/AST/DeclTemplate.cpp

@@ -1463,7 +1463,7 @@ SourceRange VarTemplateSpecializationDecl::getSourceRange() const {
     assert(!Pattern.isNull() &&
            "Variable template specialization without pattern?");
     if (const auto *VTPSD =
-            Pattern.dyn_cast<VarTemplatePartialSpecializationDecl *>())
+            dyn_cast<VarTemplatePartialSpecializationDecl *>(Pattern))
       return VTPSD->getSourceRange();
     VarTemplateDecl *VTD = cast<VarTemplateDecl *>(Pattern);
     if (hasInit()) {