[clang] [Feat] Allow Finding across only parts of an AST. (PR #127423)

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matts1 wrote:

It's a general performance optimization. For example, you could write a predicate to check whether it's in the std namespace to avoid checking the standard library, or a predicate to check whether a class is a subclass of `proto.Message` to skip generated protobuf code.

https://github.com/llvm/llvm-project/pull/127423