[analyzer] Use makeNode instead of ExplodedGraph::getNode (#218462)
The method `CoreEngine::makeNode` is the canonical way of creating a new
node in the exploded graph and connecting it to its predecessor. Apply
it in two locations that previously duplicated its logic.
Note that `ExplodedGraph::getNode` always returns a non-null
`ExplodedNode *` (that points to either an old node or the freshly
created node); `inlineCall` had no reason to check whether it returns a
nullpointer.
This change is very close to being NFC, but could technically change the
behavior if the state is `PosteriorlyOverconstrained` (which is
vanishingly rare).