[PERF] Optimize 3rd heaviest func, (81b -> 10m) (#15043)

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.

What I can get behind is via `strlen_on_c_strings` changing from 31
million instructions into 76k. 🎉 🥳

changelog: [`strlen_on_c_strings`]: Optimize it by 99.75%

clippy_utils/src/lib.rs

@@ -1793,10 +1793,9 @@ pub fn in_automatically_derived(tcx: TyCtxt<'_>, id: HirId) -> bool {
 
 /// Checks if the given `DefId` matches the `libc` item.
 pub fn match_libc_symbol(cx: &LateContext<'_>, did: DefId, name: Symbol) -> bool {
-    let path = cx.get_def_path(did);
     // libc is meant to be used as a flat list of names, but they're all actually defined in different
     // modules based on the target platform. Ignore everything but crate name and the item name.
-    path.first().is_some_and(|s| *s == sym::libc) && path.last().copied() == Some(name)
+    cx.tcx.crate_name(did.krate) == sym::libc && cx.tcx.def_path_str(did).ends_with(name.as_str())
 }
 
 /// Returns the list of condition expressions and the list of blocks in a