[clang-tools-extra] [clang-tidy] Add new check 'misc-scope-reduction' (PR #175429)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

If AI was used, please disclose it. And to what extent it was used.

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Please remove unnecessary comment such as `// early exit`.

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>From #22366, I think there is another option for the name of this check: misc-localizing-variables. But I'm neutral with these names.

What about put this check in "readability" module instead of "misc"?

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I wonder if we can use `RecursiveASTVisitor`/`DynamicRecursiveASTVisitor` to find the innermost scope containing all `DeclRefExpr`.

https://github.com/llvm/llvm-project/pull/175429