To reduce the overheads of copying BSONObj into MatchExpressions, we aim to remove the _backingBSON from the MatchExpression. The MatchExpression will compare against a reference of an already existing BSONObj which survives the query. 

Removing _backingBSON, this generates errors in jstests/aggregation/sources/lookup/lookup_foreign_collation.js

e..g, patch

This ticket shall address those errors.