Adjust tests/ui/attr-usage-inline.rs

- Move `tests/ui/attr-usage-inline.rs` to `tests/ui/attributes/inline/`.
- Briefly document test intent.
- Drop unnecessary `#[allow(dead_code)]` as this is allowed-by-default
  for ui test suite.

Author: jieyouxu

tests/ui/attr-usage-inline.rs renamed to tests/ui/attributes/inline/attr-usage-inline.rs

@@ -1,4 +1,5 @@
-#![allow(dead_code)]
+//! Check that `#[inline]` attribute can only be applied to fn-like targets (e.g. function or
+//! closure), and when misapplied to other targets an error is emitted.
 
 #[inline]
 fn f() {}

tests/ui/attr-usage-inline.stderr renamed to tests/ui/attributes/inline/attr-usage-inline.stderr

@@ -1,13 +1,13 @@
 error[E0518]: attribute should be applied to function or closure
-  --> $DIR/attr-usage-inline.rs:6:1
+  --> $DIR/attr-usage-inline.rs:7:1
    |
 LL | #[inline]
    | ^^^^^^^^^
 LL | struct S;
    | --------- not a function or closure
 
 error[E0518]: attribute should be applied to function or closure
-  --> $DIR/attr-usage-inline.rs:20:1
+  --> $DIR/attr-usage-inline.rs:21:1
    |
 LL | #[inline]
    | ^^^^^^^^^ not a function or closure