Code complete does not work with `cfg_attr` attribute

Code complete does not work with the cfg_attr attribute, but it works fine with the cfg attribute.

1. Code complete for predicate

#[cfg_attr(target_arch = "x86_64", inline(always))]
//         ^^^^^^^^^^^^^^^^^^^^^^ // code complete for predicate
fn test() {}

2. Code complete for attr in cfg_attr

#[cfg_attr(target_arch = "x86_64", inline(always))]
//                                 ^^^^^^^^^^^^^^   // code complete for attr in cfg_attr
fn test() {}

Reproduce the two code-completion cases in the issue: the predicate and attribute positions in a Rust cfg_attr expression, comparing them with cfg. Trace the code-completion entry point for attributes and verify that both positions provide completion results for cfg_attr.