perf: bail early in too_many_lines lint if rule is not enabled. (#17605)

If the rule isn't enabled (which is the default behavior), there's no
reason to scan the lines in the given function and count them. This
avoids extra work for most users.

changelog: none

Author: rustbot

clippy_lints/src/functions/too_many_lines.rs

@@ -1,4 +1,5 @@
 use clippy_utils::diagnostics::span_lint;
+use clippy_utils::is_lint_allowed;
 use clippy_utils::source::SpanExt as _;
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
@@ -22,6 +23,10 @@ pub(super) fn check_fn(
         return;
     }
 
+    if is_lint_allowed(cx, TOO_MANY_LINES, cx.tcx.local_def_id_to_hir_id(def_id)) {
+        return;
+    }
+
     let mut line_count: u64 = 0;
     let too_many = body.value.span.check_text(cx, |src| {
         let mut in_comment = false;