05843dc6ab changed the serialization of the body
of LambdaExpr to avoid a mutation in LambdaExpr::getBody and to avoid a missing
body in LambdaExpr::children.
Unfortunately this replaced one bug by another: we are now duplicating the body
during deserialization; that is after deserialization the identity:
E->getBody() == E->getCallOperator()->getBody() does not hold.
Fix that by instead lazily loading the body from the call operator when needed.
Differential Revision: https://reviews.llvm.org/D83009
Reviewed By: martong, aaron.ballman, vabridgers
Bruno Ricci