Rollup merge of #131034 - Urgau:cfg-true-false, r=nnethercote

Implement RFC3695 Allow boolean literals as cfg predicates

This PR implements https://github.com/rust-lang/rfcs/pull/3695: allow boolean literals as cfg predicates, i.e. `cfg(true)` and `cfg(false)`.

r? `@nnethercote` *(or anyone with parser knowledge)*
cc `@clubby789`

compiler/rustc_span/src/symbol.rs

@@ -544,6 +544,7 @@ symbols! {
         cfg_accessible,
         cfg_attr,
         cfg_attr_multi,
+        cfg_boolean_literals,
         cfg_doctest,
         cfg_eval,
         cfg_fmt_debug,