[clang] [analyzer] Use makeNode instead of ExplodedGraph::getNode (PR #218462)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

Now that https://github.com/llvm/llvm-project/pull/218472 was merged, this is a straightforward little change that mostly just simplifies the code, but could theoretically improve the behavior of the analyzer if a `PosteriorlyOverconstrained` state hits one of these two locations.

https://github.com/llvm/llvm-project/pull/218462