feat: option to ignore borrowing annotations completely

[hargoniX]

This PR adds support for ignoring user defined borrow annotations. This can be useful when defining
extern/export pairs as the extern might be infected by borrow annotations while in export
they are already ignored.

[hargoniX]

!bench

[leanprover-radar]

Benchmark results for d62c43a against d3db436 are in. There are no significant changes. @hargoniX

• 🟥 build//instructions: +773.8M (+0.01%)

Small changes (4🟥)

• 🟥 build/module/Lean.Compiler.IR.AddExtern//instructions: +32.4M (+3.11%) (reduced significance based on *//lines)
• 🟥 build/module/Lean.Compiler.LCNF.ToDecl//instructions: +141.4M (+3.69%) (reduced significance based on *//lines)
• 🟥 build/module/Lean.Compiler.Options//instructions: +20.2M (+3.55%)
• 🟥 elab/simp_bubblesort_256//wall-clock: +32ms (+3.28%)

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d3db4368d499ae0933d253e4771cc187b592e577 --onto e8048291010c815d0d30926924e7ad7afc18b1c0. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-11 16:57:18)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase d3db4368d499ae0933d253e4771cc187b592e577 --onto ff6816a854ef5cf2966519452d03933785f2fdeb. You can force reference manual CI using the force-manual-ci label. (2026-03-11 16:57:21)