Fixes #153545.

When a closure captures a variable whose type involves a named lifetime from the parent function (e.g., 'a), the borrow checker's region renumbering creates a separate RegionVid for each occurrence of the erased lifetime in the closure's upvar type. These new RegionVids have no external_name, so diagnostics fall back to synthetic names like '1.

This PR recovers the original lifetime name by matching the closure's upvar type against the parent function's parameter type (obtained via liberate_late_bound_regions). Both types have the same structure but different region representations, the parent has ReLateParam/ReEarlyParam with real names, the closure has ReVar from renumbering. Walking them in parallel with for_each_free_region lets us find which named lifetime corresponds to the anonymous RegionVid.

Before:

error[E0716]: temporary value dropped while borrowed
 --> test.rs:11:21
  |
6 | fn apply<'a>(
  |          -- lifetime `'1` defined here

After:

error[E0716]: temporary value dropped while borrowed
 --> test.rs:11:21
  |
6 | fn apply<'a>(
  |          -- lifetime `'a` defined here

The fix gracefully falls back to the existing '1 naming when it can't match, nested closures, local variables (not parameters), destructuring patterns, or partial captures where the type structures diverge.