Rollup merge of #55777 - nnethercote:less-P-in-ast, r=petrochenkov

Use `Lit` rather than `P<Lit>` in `ast::ExprKind`.

Because it results in fewer allocations and small speedups on some
benchmarks.

diff --cc src/libsyntax/ext/quote.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge