Auto merge of #51072 - petrochenkov:ifield, r=eddyb

Use `Ident`s for fields in HIR

Continuation of https://github.com/rust-lang/rust/pull/49718, part of https://github.com/rust-lang/rust/issues/49300

diff --cc src/libproc_macro/lib.rs
Simple merge

diff --cc src/librustc/hir/lowering.rs
Simple merge

diff --cc src/librustc_typeck/check/mod.rs
Simple merge

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

diff --cc src/libsyntax/print/pprust.rs
Simple merge