Rollup merge of #37695 - estebank:unescaped-curly, r=alexcrichton

On fmt string with unescaped `{` note how to escape

On cases of malformed format strings where a `{` hasn't been properly escaped, like `println!("{");`, present a NOTE explaining how to escape the `{` char.

Fix #34300.

diff --cc src/libsyntax_ext/format.rs
Simple merge