Suppress unstable-trait notes under `-Zforce-unstable-if-unmarked` - Fixes https://github.com/rust-lang/rust/issues/152692. --- https://github.com/rust-lang/rust/pull/151036 adds extra diagnostic text (“the nightly-only, unstable trait”) to note when a not-implemented trait is unstable. However, that extra text is usually unhelpful when building a crate graph with `-Zforce-unstable-if-unmarked` (such as the compiler or stdlib), because *any* trait not explicitly marked stable will be treated as unstable. (For typical compiler contributors using the stage0 compiler, this fix won't take effect until the next bootstrap beta bump.)