rustfmt: Also allow bool literals as first item of let chain This is a functional cherry-pick of https://github.com/rust-lang/rustfmt/pull/6492 I'm bringing this change over directly as the subtree sync is taking more effort than anticipated (some unrelated r-l/rustfmt changes need to be reverted before we perform the full sync) and we need to ensure that rustfmt behavior accounts with the final style guide rules as part of let chain stabilization. r? ````@ghost````