[libclang] Migrate away from PointerUnion::dyn_cast (NFC) (#125631)

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 Storage to be nonnull.  Note that if Storage were
null, dereferencing Ovl would trigger a segfault.

Author: kazutakahirata

clang/tools/libclang/CIndex.cpp

@@ -5370,12 +5370,12 @@ CXString clang_getCursorSpelling(CXCursor C) {
 
     case CXCursor_OverloadedDeclRef: {
       OverloadedDeclRefStorage Storage = getCursorOverloadedDeclRef(C).first;
-      if (const Decl *D = Storage.dyn_cast<const Decl *>()) {
+      if (const Decl *D = dyn_cast<const Decl *>(Storage)) {
         if (const NamedDecl *ND = dyn_cast<NamedDecl>(D))
           return cxstring::createDup(ND->getNameAsString());
         return cxstring::createEmpty();
       }
-      if (const OverloadExpr *E = Storage.dyn_cast<const OverloadExpr *>())
+      if (const OverloadExpr *E = dyn_cast<const OverloadExpr *>(Storage))
         return cxstring::createDup(E->getName().getAsString());
       OverloadedTemplateStorage *Ovl =
           cast<OverloadedTemplateStorage *>(Storage);