[clang-tidy] Fix false negatives in performance-faster-string-find with libstdc++ (#185559)

The check previously used a custom, heuristic-based matcher to avoid
matching inside uninstantiated generic templates. However, the matcher
caught the `SubstTemplateTypeParmType` from `libstdc++`, silencing valid
warnings.

This PR removes manual template filtering and uses
`TK_IgnoreUnlessSpelledInSource` instead.

As of AI Usage: Gemini 3 is used for pre-commit reviews.
Closes https://github.com/llvm/llvm-project/issues/182012

GitOrigin-RevId: 3cd9ca39787d7eb380b24ac60b32e7f24bdbce1e