feat(frontends/lean/elaborator): tolerate orphan bounded variables
feat(frontends/lean/elaborator): tolerate orphan bounded variables

The pretty printer is used to display ill-formed expressions,
and it uses type_context.

fpvandoorn/lean,Kha/lean,dselsam/lean,dselsam/lean,leodemoura/lean,avigad/lean,Kha/lean,leanprover/lean,leanprover-community/lean,rlewis1988/lean,leodemoura/lean,leanprover-community/lean,avigad/lean,leanprover/lean,rlewis1988/lean,johoelzl/lean,fpvandoorn/lean,johoelzl/lean,leanprover/lean,johoelzl/lean,digama0/lean,digama0/lean,fpvandoorn/lean,leanprover/lean,johoelzl/lean,leanprover/lean,johoelzl/lean,digama0/lean,avigad/lean,leanprover-community/lean,leanprover-community/lean,rlewis1988/lean,dselsam/lean,rlewis1988/lean,leanprover-community/lean,dselsam/lean,Kha/lean,dselsam/lean,Kha/lean,leodemoura/lean,avigad/lean,johoelzl/lean,rlewis1988/lean,fpvandoorn/lean,avigad/lean,leodemoura/lean,leanprover/lean,Kha/lean,dselsam/lean,leodemoura/lean,fpvandoorn/lean,digama0/lean,leanprover-community/lean,leodemoura/lean,rlewis1988/lean,fpvandoorn/lean,digama0/lean,avigad/lean,Kha/lean,digama0/lean