Implement #[rustc_default_body_unstable]

This attribute allows to mark default body of a trait function as
unstable. This means that implementing the trait without implementing
the function will require enabling unstable feature.

This is useful in conjunction with `#[rustc_must_implement_one_of]`,
we may want to relax requirements for a trait, for example allowing
implementing either of `PartialEq::{eq, ne}`, but do so in a safe way
-- making implementation of only `PartialEq::ne` unstable.

compiler/rustc_span/src/symbol.rs

@@ -1215,6 +1215,7 @@ symbols! {
         rustc_const_unstable,
         rustc_conversion_suggestion,
         rustc_def_path,
+        rustc_default_body_unstable,
         rustc_diagnostic_item,
         rustc_diagnostic_macros,
         rustc_dirty,