[MemProf] Remove context id set from nodes and recompute on demand (#94415)

The ContextIds set on the ContextNode struct is not technically needed
as we can compute it from either the callee or caller edge context ids.
Remove it and add a helper to recompute from the edges on demand. Also
add helpers to compute the node allocation type and whether the context
ids are empty from the edges without needing to first compute the node's
context id set, to minimize the runtime cost increase.

This yielded a 20% reduction in peak memory for a large thin link, for
about a 2% time increase (which is more than offset by some other recent
time efficiency improvements).

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