Fifth Edition has not gone into detail the way previous editions have as to the exact biology of dragonkind. However, I have sitting next to me the AD&D Monster Manual, the AD&D Council of Wyrms Campaign Option, and the 3e Draconomicon. Assuming this old lore holds, dragons can eat anything if they must. Including, explicitly, rocks and dirt. They also need remarkably little water, generally not drinking water at all if they eat living animals with blood.
Red dragons in particular tend to lair in high mountains, so a cave-in will not take them long to clear (we're not talking about miles of caved-in tunnels here), and rain or melted snow should seep through the cave-in eventually.
Red dragons also tend to have servants, who qualify as an emergency sustenance source. Assuming none of those servants could create water, their blood will suffice. The dragon is no real danger from a cave-in, even if it is much younger than adult. An adult red will bash and melt its way out in a matter of hours, tops. Some servants will get eaten, but that's more a rage side-effect than a necessity, and an expected result of serving a red dragon.
A very young red might not have the servants to last a season or the strength to get out on its own. If there's not an older dragon who checks on it periodically (or that period is really long - old dragons might consider a once-per-decade check-in to be timely), the hatchling might die in this case.... its best bet, if it is able, is to go into hibernation. Eat everything edible in the lair, pile everything into the horde, and sleep atop the gold and statues in the hope of waking in a few years, bigger and stronger than before.
I'll also note that traditional true dragons from previous editions (not sure about 4e) have always had spellcasting abilities as a result of dragonkind's intense magical nature. Whether an individual dragon could cast create water was a matter of chance, but the only AD&D or 3e dragons without the ability to cast some kind of magic were gem dragons in the Council of Wyrms setting that chose to become pure psionicists, forsaking their innate magic to enhance their psychic abilities. Metallic and chromatic dragons had the option to forsake their innate magic as well, but only to become pure magi or clerics, so they still had magic of some sort