Optimize by 99.75% strlen_on_c_strings, 8.5% globally

So this is funny, the query `tcx.module_children` was top 3 in most
time consuming functions in Clippy, it was being called 24384 times in
tokio. "Unacceptable!" I thought. Digging a bit around, turns out that
`clippy::strlen_on_c_strings` was calling for `get_def_path` via
`match_libc_symbol`. This query pretty-prints things and performs some
analysis.

Yes, we were running early lint checks to see if symbols were from `libc`.
I don't really trust callgrind when it says I've turn 81 billion instructions
into like 10 million. So I benchmarked this the good ol' "compiling 20 times
without incr" method and it went from 0.31s-0.45s to 0.25s constistently.

(Profiled, and "benchmarked") on tokio.

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