Rollup merge of #41957 - llogiq:clippy-libsyntax, r=petrochenkov

Fix some clippy warnings in libsyntax

This is mostly removing stray ampersands, needless returns and lifetimes. Basically a lot of small changes.

diff --cc src/libsyntax/ext/tt/macro_parser.rs
Simple merge

diff --cc src/libsyntax/json.rs
Simple merge

diff --cc src/libsyntax/parse/lexer/mod.rs
Simple merge

diff --cc src/libsyntax/visit.rs
Simple merge