feat(frontends/lean/elaborator): generate type information for placeholders
feat(frontends/lean/elaborator): generate type information for placeholders

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

leanprover/lean,leodemoura/lean,johoelzl/lean,leanprover/lean,dselsam/lean,dselsam/lean,levnach/lean,fgdorais/lean,soonhokong/lean-osx,fgdorais/lean,levnach/lean,rlewis1988/lean,digama0/lean,UlrikBuchholtz/lean,leanprover/lean,leanprover-community/lean,dselsam/lean,javra/lean,soonhokong/lean-windows,fpvandoorn/lean,htzh/lean,soonhokong/lean-osx,Kha/lean,digama0/lean,johoelzl/lean,javra/lean,soonhokong/lean,htzh/lean,eigengrau/lean,avigad/lean,fpvandoorn/lean,soonhokong/lean,leanprover-community/lean,avigad/lean,rlewis1988/lean,dselsam/lean,digama0/lean,javra/lean,fpvandoorn/lean,Kha/lean,eigengrau/lean,htzh/lean,soonhokong/lean-osx,soonhokong/lean-windows,leodemoura/lean,soonhokong/lean-windows,leodemoura/lean,soonhokong/lean,soonhokong/lean-windows,fpvandoorn/lean2,fpvandoorn/lean2,Kha/lean,rlewis1988/lean,UlrikBuchholtz/lean,UlrikBuchholtz/lean,javra/lean,johoelzl/lean,sp3ctum/lean,digama0/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,fgdorais/lean,leodemoura/lean,leanprover-community/lean,Kha/lean,digama0/lean,soonhokong/lean-windows,johoelzl/lean,soonhokong/lean,rlewis1988/lean,sp3ctum/lean,leodemoura/lean,sp3ctum/lean,leanprover-community/lean,levnach/lean,avigad/lean,c-cube/lean,fpvandoorn/lean2,fpvandoorn/lean2,levnach/lean,Kha/lean,fpvandoorn/lean,leanprover/lean,avigad/lean,c-cube/lean,eigengrau/lean,leanprover-community/lean,Kha/lean,javra/lean,soonhokong/lean-osx,fpvandoorn/lean,dselsam/lean,avigad/lean,rlewis1988/lean,fgdorais/lean,leodemoura/lean,UlrikBuchholtz/lean,c-cube/lean,avigad/lean,johoelzl/lean,leanprover/lean,leanprover/lean,c-cube/lean,htzh/lean,dselsam/lean,soonhokong/lean,johoelzl/lean,htzh/lean,soonhokong/lean-osx,levnach/lean,digama0/lean,sp3ctum/lean,fpvandoorn/lean,rlewis1988/lean,eigengrau/lean,fgdorais/lean,fgdorais/lean,sp3ctum/lean,c-cube/lean,eigengrau/lean,leanprover-community/lean