Rollup merge of #120139 - compiler-errors:fnonce-shim, r=BoxyUwU

Do not normalize closure signature when building `FnOnce` shim

It is not necessary to normalize the closure signature when building an `FnOnce` shim for an `Fn`/`FnMut` closure. That closure shim is just calling `FnMut::call_mut(&mut self)` anyways.

It's also somewhat sketchy that we were ever doing this to begin with, since we're normalizing with a `ParamEnv::reveal_all()` param-env, which is definitely not right with possibly polymorphic substs.

This cuts out a tiny bit of unnecessary work in `Instance::resolve` and simplifies the signature because now we can unconditionally return an `Instance`.

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -435,7 +435,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                     args,
                                     ty::ClosureKind::FnOnce,
                                 )
-                                .expect("failed to normalize and resolve closure during codegen")
                                 .polymorphize(bx.cx().tcx());
                                 OperandValue::Immediate(bx.cx().get_fn_addr(instance))
                             }