Auto merge of #143290 - azhogin:azhogin/link-pub-async-impls, r=oli-obk

pub async fn impl is monomorphized when func itself is monomorphized

Implentation coroutine (`func::{closure#0}`) is monomorphized, when func itself is monomorphized.

Currently, when `pub async fn foo(..)` is exported from lib and used in several dependent crates, only 'header' function is monomorphized in the defining crate. 'header' function, returning coroutine object, is monomorphized, but the coroutine's poll function (which actually implements all the logic for the function) is not. In such situation, `func::{closure#0}` will be monomorphized in every dependency.

This PR adds monomorphization for `func::{closure#0}` (coroutine poll function), when func itself is monomorphized.

Simple test with one lib async function and ten dependent crates (executable) that use the function, shows 5-7% compilation time improvement (single-threaded).

Author: bors