[clang] [NFC][analyzer] Remove class 'NodeBuilder' (PR #217319)

[Balázs Benics via cfe-commits]

steakhal wrote:

Let me forward couple seemingly relevant comments:

  - CoreEngine::makeNode is const, so the existing const CoreEngine &getCoreEngine() const already suffices for addTransitionImpl (that's how NodeBuilder::generateNode reached it). Is
  the non-const overload deliberate — signalling that node creation mutates the graph — or can it be dropped so CheckerContext.h doesn't expose a mutable CoreEngine to every checker?
  - ExplodedNode still has friend class NodeBuilder; (ExplodedGraph.h:69), keeping the name alive as an incomplete type. Delete it here? Look for similar stale spellings.
  - The currBldrCtx comment block and now-unused getBuilderContext() — fold into this PR, or leave to the announced NodeBuilderContext follow-up?
  - On your own EndFunction question: the asymmetry looks worth fixing, but as a separate non-NFC change with a test — please keep it out of this PR.

https://github.com/llvm/llvm-project/pull/217319