[clang][Serialization] Don't duplicate the body of LambdaExpr during deserialization

05843dc6 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