Allow approx-constant in x clippy library

Author: flip1995

src/bootstrap/src/core/build_steps/clippy.rs

@@ -19,6 +19,7 @@ const IGNORED_RULES_FOR_STD_AND_RUSTC: &[&str] = &[
     "too_many_arguments",
     "needless_lifetimes", // people want to keep the lifetimes
     "wrong_self_convention",
+    "approx_constant", // libcore is what defines those
 ];
 
 fn lint_args(builder: &Builder<'_>, config: &LintConfig, ignored_rules: &[&str]) -> Vec<String> {