Rollup merge of #100559 - nnethercote:parser-simplifications, r=compiler-errors

Parser simplifications

Best reviewed one commit at a time.

r? ``@compiler-errors``

diff --cc compiler/rustc_parse/src/parser/diagnostics.rs
Simple merge

diff --cc compiler/rustc_parse/src/parser/expr.rs
Simple merge

diff --cc compiler/rustc_parse/src/parser/stmt.rs
Simple merge