Avoid ICE when referencing desugared local binding in borrow error

[estebank]

To avoid leaking the names of local bindings from expressions like for loops, #60984 explicitly ignored them, but an assertion that LocalKind::Var must have a name would trigger an ICE.

Before this change, the binding generated by desugaring the for loop would leak into the diagnostic (#63027):

error[E0515]: cannot return value referencing local variable `__next`
  --> return-local-binding-from-desugaring.rs:LL:CC
   |
LL |     for ref x in xs {
   |         ----- `__next` is borrowed here
...
LL |     result
   |     ^^^^^^ returns a value referencing data owned by the current function

Ideally LocalKind would carry more information to more accurately explain the problem, but for now, in order to avoid the ICE (fix #63026), we accept LocalKind::Var without a name and produce the following output:

error[E0515]: cannot return value referencing local binding
  --> $DIR/return-local-binding-from-desugaring.rs:30:5
   |
LL |     for ref x in xs {
   |                  -- local binding introduced here
...
LL |     result
   |     ^^^^^^ returns a value referencing data owned by the current function

[rust-highfive]

r? @zackmdavis

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[estebank]

cc @matthewjasper

[matthewjasper]

r? @matthewjasper

[matthewjasper]

@bors r+

[bors]

📌 Commit 01ba0e3 has been approved by matthewjasper