[clang] [clang][Sema] Fix the continue and break scope for while loops (PR #152606)

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Sirraide wrote:

@ojhunt FYI, I think would be easier to add an extra flag to the `Scope` class that basically keeps track of whether break/continue are currently allowed and only set that right before we start parsing the body. This would be different from the BreakScope/ContinueScope flags in that in e.g. `while(({ breka; true;})) {}`, we *do* want the enclosing `while` loop to be the parent `ContinueScope`—we just don’t want to actually allow `continue`/`break` in it yet.

So my suggestion would be to add that flag to Scope and then in `Sema::ActOnContinueStmt()` and `Sema::ActOnContinueStmt()`, we check if the parent continue/break scope has that flag set and issue a diagnostic if not.

I don’t think changes to codegen should be required at all since this refactor shouldn’t introduce any new valid code patterns—it should just disallow existing ones.

https://github.com/llvm/llvm-project/pull/152606