This PR prevents `rcases` and `obtain` from creating absurdly long case
tag names when taking single constructor types (like `Exists`) apart.
Fixes#6550
The change does not affect `cases` and `induction`, it seems (where the
user might be surprised to not address the single goal with a name),
because I make the change in Lean/`Meta/Tactic/Induction.lean`, not
`Lean/Elab/Tactic/Induction.lean`. Yes, that's confusing.
