[clang] [analyzer] Introduce the check::BlockEntrance checker callback (PR #140924)

[Balázs Benics via cfe-commits]

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@@ -548,6 +564,8 @@ class CheckerProgramPointTag : public SimpleProgramPointTag {
 template <typename CHECK1, typename... CHECKs>
 class Checker : public CHECK1, public CHECKs..., public CheckerBase {
 public:
+  using BlockEntrance = clang::BlockEntrance;
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balazs-benics-sonarsource wrote:

> Note that I'm planning to merge my CheckerFamily PR https://github.com/llvm/llvm-project/pull/139256 very soon and it will refactor this area -- will you be able to rebase your commit to that revision?

As I was thinking about this now, I think I'd prefer landing this first.
I'd need to backport this into clang-20, and that would make it (marginally) more complicated.
I could rebase yours if this would be the case.

But I'm also fine with merging yours first, rebase this one, then backport and resolve conflicts for our internal fork.
My primary objective is to merge this PR tomorrow at latest.

https://github.com/llvm/llvm-project/pull/140924