Rollup merge of #107551 - fee1-dead-contrib:rm_const_fnmut_helper, r=oli-obk

 Replace `ConstFnMutClosure` with const closures

Also fixes a parser bug. cc `@oli-obk` for compiler changes

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

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