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

compiler/rustc_parse/src/parser/expr.rs

@@ -2109,7 +2109,7 @@ impl<'a> Parser<'a> {
             ClosureBinder::NotPresent
         };
 
-        let constness = self.parse_constness(Case::Sensitive);
+        let constness = self.parse_closure_constness(Case::Sensitive);
 
         let movability =
             if self.eat_keyword(kw::Static) { Movability::Static } else { Movability::Movable };