Rollup merge of #138844 - petrochenkov:cfgtrace2, r=nnethercote

expand: Leave traces when expanding `cfg` attributes

This is the same as https://github.com/rust-lang/rust/pull/138515, but for `cfg(true)` instead of `cfg_attr`.

The difference is that `cfg(true)`s already left "traces" after themselves - the `cfg` attributes themselves, with `expanded_inert_attrs` set to true, with full tokens, available to proc macros.
This is not a reasonably expected behavior, but it could not be removed without a replacement, because a [major rustdoc feature](https://github.com/rust-lang/rfcs/pull/3631) and a number of clippy lints rely on it. This PR implements a replacement.

This needs a crater run, because it changes observable behavior (in an intended way) - proc macros can no longer see expanded `cfg(true)` attributes.

(Some minor unnecessary special casing for `sym::cfg_attr` is also removed in this PR.)

r? `@nnethercote`

compiler/rustc_span/src/symbol.rs

@@ -623,6 +623,7 @@ symbols! {
         cfg_target_has_atomic_equal_alignment,
         cfg_target_thread_local,
         cfg_target_vendor,
+        cfg_trace: "<cfg>", // must not be a valid identifier
         cfg_ub_checks,
         cfg_version,
         cfi,