[NFC][clang-tidy]improve performance for misc-unused-using-decls check (#78231)

`UnusedUsingDeclsCheck::removeFromFoundDecls` will be called with high
frequency. At current time it will check every `Context`.
This patch adds a cache to reduce algorithm complexity.

GitOrigin-RevId: dca6f60bcdd4a8e6c5defad454d9c470a27701fe